Chapter X.
ALGAE CONTROL
Undesirable algal growth is probably the number one problem that hobbyists have in maintaining planted aquariums (or any aquarium for that matter). I suspect that many aquarists ultimately give up on keeping planted aquariums because of their frustration in trying to combat uninhibited algal growth.
Q. My 55 gal tank is plagued with an algae that is black and velvety, like black fur. It is very tenacious. About every other month, I have to scrub the algae off of the bogwood. I have been keeping fish since 1956 and have picked up this very difficult algae only in the past 5 years. (The substrate is a small gravel.)
A. I’ve had few problems with algae in my planted tanks since I started (in 1987) using soil substrates, adequate lighting, and lots of different plant species. I’ve asked fellow hobbyists to give me their worst algae to test my theory that good plant growth will control any algae. These test algae usually spread a little and grow for awhile. After a year or so, though, they seem to just disappear or hang on at manageable levels. It doesn’t matter whether the algae is ‘black fur’, ‘blue-green’, ‘green-water’, or ‘green mat’ algae.
I would focus on increasing total plant growth in your tank. First, gravel is a very poor substrate for rooted aquatic plants. Second, I would make sure the lighting is adequate. Third, I would consider adding emergent plants, even if it is only duckweed, to your tank. Once the tank has the combination of good growing conditions and many fast-growing plants, plants should prevent algal takeovers.
Unfortunately, most hobbyists see plants only as decoration; they have not learned to use plants to control algae.
A.Common Methods for Controlling Algae
1.Algaecides, Chlorox, and Antibiotics
Algaecides, which are chemicals that kill algae, often cause more problems than they solve in planted aquariums. The active ingredient of almost all common algaecides is either copper or simazine. Both are toxic to fish and plants [1,2]. The dose that will kill algae in an aquarium without harming fish or plants is often hard– if not impossible– to determine. Even if the algaecide doesn’t kill the fish, the dead algae sometimes will. Dying algae may release toxins into the water or its decomposition may remove oxygen from the water. Thus, it is not uncommon for fish to die when algae is abruptly killed.
Chlorox is a sterilizing agent that is sometimes used by botanists to remove filamentous algae from plants that will be used in short-term experiments. A few experienced hobbyists routinely dip the stem tops of new plants in a 1:20 dilution of ordinary household bleach for a few minutes to kill attached algae. This method works but should be used with care, as it may kill delicate plants or endanger fish if the chlorox gets into the aquarium itself.
Q. I am having a major problem with algae in my tank. Do you think I should add copper sulfate?
A. No, because copper is toxic. What’s more, it is virtually impossible to predict a safe copper level. There are just too many variables that affect copper toxicity (see page 14). Thus, you can’t safely predict if you add X amount of copper to X amount of water that the copper will kill algae but not harm the plants or fish. Even if the copper doesn’t kill the fish, it may prevent them from spawning, inhibit their growth rate, or harm them in other insidious ways.
Some hobbyists, desperate to control green-water algae, have tried flocculents such as alum, which are sometimes sold as water-clearing agents. These products should never be added to home aquariums, or at least those containing fish. Flocculents are positively charged compounds that non-specifically bind negatively charged particles, so that they clump and precipitate out of solution, thereby clearing the water. Because the membranes of algae cells have a negative charge, flocculents will indeed remove green-water algae from the water. [The mechanism of flocculation is the same as that for chemically removing soil particles from water (see page 135).] The problem is that the gill surfaces of fish also carry a negative charge; flocculents readily bind the delicate gill filaments together, destroying gill structure and function [3,4].
Q: I am battling green water algae in my 40 gal tank. After I change water or use a diatom filter, the water looks clear for a day or two. By the second day, though, the green water algae comes back.
I have also tried several times at least two different aquarium brands of flocculents. They work. The problem, though, is that they seem to stress the fish, especially the tetras. (I have lost about 10 green neons, among other fish.)
A: If a chemical is potent enough to flocculate (or clump) green water algae, then it could flocculate the delicate gill membranes of fish. Both gill membranes and algae cells have a negative charge that makes them susceptible to flocculants.
Adding floating plants to the tank or using a UV sterilizing filter are effective and less harmful methods for combatting green-water algae.
Less objectionable are antibiotics and UV sterilizing filters. Erythromycin and kanamycin kill many blue-green algae when used at about half the dosage recommended for treating diseased fish. (Blue-green algae are actually cyanobacteria and share enough characteristics with gram-positive bacteria to make them sensitive to these two antibiotics.) Also, UV sterilizing filters will kill green-water algae without harming the other tank inhabitants. (The algae are killed as they pass next to the UV lamp within the sterilizing filter.)
The home aquarium is an ecosystem. I would advise against using chemicals (e.g., copper sulfate) that kill indiscriminately.
2.Light Reduction
Algae are similar to submerged plants in that they can use only a fraction of full sunlight and are harmed by high light intensities; most algae are basically shade organisms [5,6]. Furthermore, many species can adapt to very low light levels (see page 162).
Most algae cannot use strong light (Table X-1). (While they may survive at higher levels, they aren’t growing any faster.) Although green algae (Chlorophyta) can use moderately intense light (211 µmol/m2/s), none of the algae listed come even close to using full sunlight (2,000 µmol/m2/s).1
Table X-1. Saturating Light for the Growth of Algae [7].
Range shown is the ‘standard error of the mean’.
| Algal Class: | Saturating Light (µmol/m2/s) |
| Bacillariophyceae (22 species) | 84 (± 8.1) |
| Chlorophyta (9 species) | 211 (±58) |
| Cyanophyceae (14 species) | 39 (±6.2) |
| Dinophyceae (17 species) | 47 (±6.6) |
| Rhodophyceae (3 species) | 79 (±20) |
Moreover, algae are inhibited by intense light, both ultraviolet and visible light. ‘Photoinhibition’ by ordinary light generally begins at about 200 µmol/m2/s, but it ranges from 86 for the Dinophyceae to 233 for the Bacillariophyceae [7].
3.Water Changes
Many hobbyists report that they have been unable to combat an entrenched algae with water changes. Indeed, I see little connection between water changes and algal growth. Well-established tanks with plants usually have few algae problems. Even though I only change the water every few months or so, there is little algae. And when algae problems occasionally arise, water changes seem quite ineffective.
Q. If algae doesn’t need much light, why do many algae seem to grow so much better in sunlight?
A. Intense light makes iron more available for algae in a process called ‘iron photoreduction’ (see page 167). Iron’s increased availability, not the intense light per se, may be what stimulates the algae.
4.Algae-Eating Fish, Shrimp, and Snails
Algae-eating fish and shrimp can be useful, especially in a new tank setup where algae problems are common. Snails also help by cleaning plant leaves of attached microorganisms and debris, thereby preventing algae from gaining a foothold [10,11].
However, depending on fish (and other organisms) to control algae may be self-defeating in the long run. This is because algae-eating fish will often rid the tank of algae they like to eat. If the tank remains out of balance, though, it is only a matter of time before less tasty algae enter the tank. No aquarium fish will eat blue-green algae and only the Siameses algae eater (Crossocheilus siamensis) will eat the black fur algae. These algae can rapidly gain a foothold when the more palatable algae are no longer in the tank to compete with them.
Q. I have a 300 gallon ‘High-tech’ tank. I fertilize the plants with plant tablets and liquid fertilizers every other day or so. Substrate is iron-rich laterite mixed with 2-3 mm gravel.
Weekly, I change about 40 gal of water and replace it with de-ionized water. Gravel is cleaned here and there, wherever there is room. There are relatively few fish in the tank and they are fed very sparingly. Nitrite level is zero, and Fe is 0.1 mg/l. Nitrates are less than 0.2 mg/l and the phosphates are 0.15 mg/l, at the most.
What is really driving me up the wall is the fact that I have had the little tuffs of the red algae and a green thread algae on rocks and older leaves. I also have a number of so-called algae-eating fish, but they have never shown me anything.
All the literature points to poor maintenance, overfeeding, high nitrates and phosphates, but as I have already pointed out, the tank is maintained religiously and both nitrates and phosphates are almost nonexistent. I also understand that there will always be algae in a healthy tank, but I have seen too many plant tanks, especially in Europe, that have none at all.
A. Although your phosphate levels are indeed very low (lower than for most aquariums), they are quite sufficient for many algae. As you have seen, trying to reduce P levels in the aquarium to levels that will eliminate algal growth is almost impossible. I would be more concerned about Fe than P (or nitrates). That 0.1 ppm Fe level may be stimulating algae.
For example, I was able to eliminate a slimy, brown algae in one of my tanks by adding several Chinese algae-eaters (Gyrinocheilus aymonieri). The algae-eaters devoured the algae within a week. But a few months later the tank was overtaken by the potentially devastating blue-green algae, which the Chinese algae-eaters would not touch.
Although I have no objection to algae-eating fish, I no longer bother keeping them for algae control.
5.Phosphate Removal
In natural waters, nutrient increases from pollution often lead to undesirable algal growth and the destruction of aquatic plants. After years of controversy between biologists and the detergent industry, it is now commonly accepted that phosphate limits algal growth in many freshwaters. The phosphate concentration in unpolluted natural waters is indeed very low, between 0.003 and 0.02 mg/l P. This limits algal growth, since only a few algal species can use less than 0.02 mg/l P [12].
However, home aquariums typically have much higher phosphate levels. My own aquariums contain about 1-5 mg/l P, which is more than sufficent for almost any algal species. Because of the continuous addition of phosphate via fishfood, it is highly unlikely that phosphate deficiency would ever limit algal growth in the typical aquarium.
B.Competition between Algae and Plants
Algae is almost always more adept than plants at using light and nutrients. It is surprising then that ponds, aquariums, and lakes containing dense plant growth often seem to have little algal growth. Investigators [13] tested this field observation experimentally by monitoring algal growth in fish ponds when they contained no plants or when they contained Elodea canadensis (Table X-2). Algae didn’t grow as well when the ponds contained Elodea. For example, in Pond A, the number of alga cells was 6,600 cells/ml without plants in the pond. When plants were added to the pond, the number of alga cells was reduced to only 430 cells/ml.
Table X-2. Effect of Elodea canadensis on ‘Green-water’ Algae in Fish Ponds [13].
| Pond | No Plants (# cells/ml) | With Plants (# cells/ml) |
| A | 6,600 | 430 |
| B | 13,000 | 1,300 |
| C | 1,700 | 460 |
| D | 3,900 | 1,000 |
Q. I am having a terrible time with green water algae in my large Koi pond. The problem is that the Koi will eat any plants I put into the pond. Another problem is that the pond is in direct sun, so the algae is very thick. How can I get rid of the algae? (As it is, I can’t even see the fish, except when I feed them.)
A. Ponds with Koi are a problem, because these fish eat just about any plant. However, you can get around this by somehow creating a place where the plants are protected. If you establish a protected area for plants, the plants will prevent algal growth.
Hobbyist’s Follow-up Letter. I built a mini-pond for plants slightly above the main pond. Water from the main pond is pumped into the mini-pond and then flows via a waterfall into the main pond with the Koi. I keep water lilies, emergent plants, and Vallisneria in this small pond. During the winter, a mat type of algae grows on the waterfall. I keep this algae, which I don’t mind, and the plants pruned regularly. The main pond is now crystal clear even during the hottest and brightest summer months; it has stayed that way for several years. My fish are doing well, and best of all I can now see them!
Hobbyist Observation. Since there has been so much discussion about algae lately, I thought I’d throw in this recent experience. One of my swordplants was rapidly being covered by a short, brushy, fur-like alga. I tried all sorts of treatments—lots of water changes (1/3 volume per week), did not feed the fish, dark periods, hydrogen peroxide. The swordplant was doing OK but not great. I finally had a period where I was too busy to do proper maintenance, and during that time one of the faster growing weeds just took over the tank, shading the sword and other undergrowth. After this happened, the sword started to grow much better! There are a lot of new leaves, none of which has algae. I know this is telling me that light was the problem, but my reducing light levels and subjecting the tank to darkouts just wasn’t getting the job done. The undergrowth is obviously getting enough light, so I’m leaving well enough alone.
My Comment: I couldn’t have said it better. The bottom line is: ‘Let the plants do the work for you.’
1.Advantages Algae have over Plants
a) Better Adaptation to Low Light
In some instances, reducing light levels in a planted tank would hurt plants more than algae. This is because the light requirements of aquatic plants are often greater than those of many algae, especially ‘green-water’ algae (Table X-3). The median light required by 7 plant species for growth was 6.1 µmol/m2/s while algae required about one-third less (1.8 µmol/m2/s). Also, the efficiency with which algae used light was found to be 7 times greater than for the plants (7.5 v. 1.1). (This greater efficiency is apparently linked to algae’s higher chlorophyll concentration and smaller cell size.)2
Table X-3. Minimum Light Required by Algae and Plants [14].
Plants were 7 submerged species including Elodea canadensis. Algae were 8-16 species of phytoplankton. Plants and algae were exposed to fluorescent light for 16 hr per day at different light intensities.
‘Low Light Growth Efficiency’ was calculated from the slope (b) of growth versus light intensity.
| Organism | Minimum Light Requirements (µmol/m2/s) | Low-Light Growth Efficiency (b) | ||
| Median | Range | Median | Range | |
| Algae | 1.8 | 0.8 - 9 | 7.5 | 0.4 - 44 |
| Plants | 6.1 | 3 - 12 | 1.1 | 0.2 - 1.8 |
b) Algal Adaptation to the Light Spectrum
Although both green algae and plants have chlorophyll, which absorbs mainly red and blue light, many algae have accessory photosynthetic pigments that allow them to better use the full light spectrum. Thus, certain siphonaceous green algae have special carotenoids that absorb green and blue-green light and contribute to photosynthesis [6]. Many red and blue-green algae readily adapt (chromatic adaptation) to light spectral changes by changing the proportions of their specialized photosynthetic pigments. For example, when Synechocystis (a blue-green alga) is grown in green light, it produces non-chlorophyll photosynthetic pigments in a 2:2:1 ratio of red, blue, and blue-gray, respectively. When the same algae is grown in red light, it produces much less red pigment, and the ratio of pigments changes to 0.4:2:1 [16].
Q. I was told that there is a certain type of fluorescent light that is better for plants than algae. Is there any evidence for algae requiring a different light spectrum than plants?
A. No. Many algae readily adapt to light spectral changes, probably more so than plants. However, theoretically speaking, tanks with light sources ‘deficient’ in blue light (e.g., ‘Cool-white’ fluorescent, incandescent bulbs, and high pressure sodium lamps) may have less algal problems. This is because UV and blue light make iron in the water more available to algae (see page 168).
Because aquatic plants don’t have these specialized pigments (e.g., phycoerythrin, phycocyanin, and siphonoxanthin), they have little (if any) chromatic adaptation [17].
Chromatic adaptation probably takes only a few days. For example, when investigators suddenly changed the lighting of one algae culture (filtered out the shorter wavelengths below 520 nm from ‘Cool-white’ fluorescent light), algal growth lagged for 3 days [18]. However, the culture apparently adjusted to the restricted light spectrum, because it was eventually able to grow at almost the same rate as algae growing in normal light.
c) Better Adaptation to High pH and Alkaline Water
Some algae appear to be better adapted to alkaline water than aquatic plants [21]. For example, in a certain canal in Lancashire (U.K.), filamentous algae (Cladophora glomerata and various Spirogyra species) have replaced Elodea canadensis. In trying to explain how this happened, investigators compared the photosynthesis rates of the algae with E. canadensis at 4 different pHs (Table X-4). At pH 6, Elodea was actually able to photosynthesize better than both algal species, producing 45 µg O2/mg chl/min. However, with increasing pH, Elodea was inhibited much more than the algae, producing only 10 µg O2/mg Chl/min at pH 8. Apparently, the algae could use bicarbonates from alkaline water better than Elodea.
Table X-4. Effect of pH on Algal and Plant Photosynthesis [15].
| Algae or Plant | Maximum Photosynthesis Rate (µg O2/mg Chl/min) | |||
| pH 6 | pH 7 | pH 8 | pH 9 | |
| Cladophora glomerata | 18 | 27 | 27 | 25 |
| Spirogyra sp. | 35 | 43 | 41 | 26 |
| Elodea canadensis | 45 | 40 | 10 | 1 |
In a competitive situation, algae can enhance its initial advantage by driving the water pH up such that aquatic plants would be even less able to obtain photosynthetic carbon.
Red algae may not have the alkaline advantage. Marine scientists report that certain species of red and brown macroalgae from the Division Rhodophyta depend primarily upon free CO2; they cannot use bicarbonates [6].
Q. I used to see black fur and brush algae (red algae) in my softwater South American cichlid tanks, but never in the Tanganyikan tanks with their crushed coral substrate and high pH. Perhaps red algae are among those algae and water plants that can only use free CO2?
A. I would agree. In my own tanks, which contain alkaline hardwater, the black fur and brush algae that I’ve purposely added to the tanks eventually die out. These same algae have plagued other hobbyist tanks, but usually their tanks had softwater and low light.
The green mat and green water algae have been much more difficult for me to get rid of. These same green algae, which have plagued other hobbyists tanks with hardwater and intense light, probably have the alkaline advantage.
Q. I seem to have a persistent problem with a ‘green mat’ algae in my tanks where I’m trying to grow Java Fern, Amazon Swordplants, Water Sprite, and Cryptocoryne. They just don’t seem to grow faster than the algae. So many times I’ll end up losing the plants when algae covers their leaves. What can I do? (Hobbyist from AZ.)
A. Arizona water has a high pH (>8), so many aquarium plants, especially those that cannot use bicarbonates, are going to have a tough time competing with algae for their carbon.
I would make sure you have plenty of plants like Vallisneria, Hornwort, and Elodea that can use bicarbonates. (I have seen Vallisneria spiralis splendidly outcompete algae in hardwater tanks with a fertile substrate and plenty of light.) Also, I would try to include emergent plants in the tank, since they can use air CO2.
d) More Efficient Uptake of Nutrients from the Water
Another advantage is that some algae may be more adept than plants at taking up nutrients from the water. For example, the filamentous alga Draparnaldia plumosa was shown to be more efficient than the aquatic plant Elodea occidentalis in taking up major nutrients N, P, Ca, and Mg (but not K) [19]. Thus, when the algae and the plant were grown together with low phosphates (0.075 mg/l P), algal growth was not affected, but plant growth was cut in half. Furthermore, P uptake was much faster for Draparnaldia than for Elodea.
Also, blue-green algae secrete a wide assortment of iron chelators (siderophores) that help them take up iron from the water [20]. Active secretion of iron chelators might give blue-green algae an advantage over plants in iron-limited environments.
Q. Why don’t you use scientific names for the aquarium algae you are discussing? For example, the red algae in softwater aquariums is usually an Audouinella species.
A. I gave up on algal taxonomy after a biologist examined some ‘green mat’ algae from my tanks. Under the microscope, the algae turned out to be a conglomerate of many separate species. The two dominant genera identified by their filamentous branching pattern and characteristic spores, were Oedogonium and Pithophora (both green algae from the Division Chlorophyta). The Oedogonium appeared to be a mixture of not one, but several species. In addition, blue-green algae Chamaesiphon and Chroococcus species appeared as small blue-green bulbs attached to the green algae filaments. Finally, there were small populations of diatoms (Division Chrysophyta) and other miscellaneous algal species within the green mat. Thus, I decided to use common, descriptive names for the algae found in aquariums.
e) Greater Species Distribution
One overlooked advantage that algae have over plants is simply that they have a greater species distribution. An aquarium only contains the plant species that a hobbyist adds to it. Those plants may or may not adapt well to tank conditions. In contrast, any algal species could be brought in initially with plants, fish, and soil or could drop in later as an airborne spore.
Some algae produce spores that are extremely tough and long lasting. For example, spores from one blue-green algae (Anabaena) were still able to germinate after 64 years [22]. Thus, a hobbyist may have great success with aquariums– often for years– until just one little spore from a new, more resilient species of algae lands in the tank.
2.Advantages Plants have over Algae
Aquatic plants have several advantages over algae. First, rooted plants can get their nutrients from the substrate, so they do not depend on water nutrients. Even in aquariums, where the water may have excessive N and P, some of the trace elements, especially iron, may only be in the substrate. Second, emergent aquatic plants can use full sun, whereas most algal species can only use a fraction of full sunlight (see pages 158-159). Third, emergent plants can use air CO2, whereas algae must use water CO2. Thus, algae have the same carbon limitations that inhibit submerged plant growth [23].
Finally, aquatic plants have much larger stores of food reserves. For example, Myriophyllum spicatum and Vallisneria americana were found to contain between 2 and 20% carbohydrate reserves during different times of the year [24]. Generally, these food reserves confer a seasonal advantage to temperate plants. For example, water lilies emerging in the early spring use energy from rhizomal stores of carbohydrates to cover the water surface with their floating leaves before the temperature and light are sufficient for algal growth [25].
C.Factors in Controlling Algae
1.Emergent Plants
Emergent and floating plants, which have the ‘aerial advantage’, are much faster growers than fully submerged plants (see page 93). Faster growth means faster removal of nutrients that can stimulate algae in aquariums. They also reduce excessive light that submerged plants don’t need and which may only be encouraging algae. Emergent plants can protect submerged plants from algae.
Q. I have a 125 gal ‘High-tech’ tank with plenty of light and CO2 fertilization. The substrate contains laterite clay. Lately, I have noticed a brown slimy coating on the leaves of the plants, and the plants don’t seem to be growing as well. Is there a way to stop this algae? The only thing that I have changed lately is removing some swordplants. Also, a few months ago, I removed all of the duckweed, because it was overrunning the tank.
A. I would have left the duckweed in. Far better that the tank be overrun by duckweed than algae. I would recommend that you add floating plants back to this tank and thin them out periodically. (I’ve found that water lettuce is a little easier to manage than duckweed.)
All submerged plants are basically shade plants; the strong lighting in this tank is basically wasted on them. But duckweed can efficiently use your high light levels.
Thus, I have always encouraged emergent plants in my aquariums. Rather than dimming lights to control algae, I prefer to keep the lighting moderately high but add floating plants and encourage emergent growth. Thus, I can increase total plant growth and the uptake of light and nutrients by aquatic plants– rather than algae– in the same volume of water. For example, when I set up an outdoor pond (50 guppies in 25 gal) in a sunny location, there was lots of ‘green-water’ algae. However, once the floating plants (mostly Water Sprite and duckweed) began to grow, the water cleared within a week or two.
Q. About a month ago I redid my 65 gal ‘High-tech’ show tank from scratch partly because of a dark brown/black coating that was covering everything in the tank.
Now another algae problem has arisen. The fastest growing plants are two thickets of Rotala indica, and I have had to repeatedly prune them back below the water line. After the most recent cutting, a blue-green film began coating the upper portion of the thickets that were pruned and is impeding new growth. The blue-green film is now spreading to the neighboring plants.
This form of algae does not appeal to my Ottocinclus or Siamensis algae-eaters. What can I do to get rid of this algae, which I read somewhere is actually a bacteria?
A. Encourage aerial growth in your tank. Plants that have access to air have an incredible advantage over both submerged plants and algae. It was not a good idea to cut your Rotala indica thickets below the water line. Cutting off your plant’s access to air injured your plants and probably contributed to your algae problem. If you can lower the water level a little to encourage renewed aerial growth of your Rotala indica, I would do so.
Q. Algae is taking over my 45 gal tank. It grows as a film in small circles on the glass, which can only be removed with a razor blade. The algae is spreading to the Anubias.
I add CO2 and change the water weekly. This tank contains 12 one-inch fish and is planted with 12 various Anubias that are doing very well. I am not using floating plants, because I don’t feel I have excess nutrients. I don’t know where I’m deficient?
A. Even though you don’t think you have excess nutrients in the water, if there is algae, then you have excess nutrients– period. (Otherwise the algae could not grow.)
Tanks containing only Anubias, which are slow growers, are subject to being overridden with algae. These plants need protection that only faster-growing plants, especially emergent plants, can provide. Thus, I keep Anubias with Amazon swordplants and Water Sprite in one tank and the delicate Cryptocoryne cordata with partially emergent stem plants and Vallisneria in another tank. Floating duckweed is in both tanks. Other hobbyists have had good luck using Water Lettuce (Pistia stratiotes) as a protective floating plant.
Q. I have a ‘Tiny tank’ set-up (5 gal) with 1 Anubias barterii, 3 Anubias nana and 3 small Cryptocoryne in which I cannot get fur algae under control. The substrate consists of 1- 1.5 inches of sifted backyard soil under 1-1.5 inches of gravel (sandblasting grit). I have 15 watts of cool-white fluorescent light over the tank. I have about 8 Ramshorn snails and 6 Malaysian Trumpet Snails, 1 Siamensis algae-eater and 5 Mollies– all for algae control. However, these measures aren’t doing the job. Any suggestions?
A. Your Tiny tank sounds just about perfect, one that I would highly recommend for a beginner’s first tank. I like the tank size, the lighting, and the soil underlayer. There’s only one problem– the slow-growing plants. This tank desperately needs some fast-growing plants, especially emergent plants. (Your letter supports my contention that you cannot depend on fish and invertebrates to control algae.)
2.Iron
Iron may limit algal growth in aquariums, if only because so many other nutrients (e.g., N and P) are so plentiful. Also, iron is the one nutrient that is required in fairly large quantities while being the least available in oxygenated water. Thus, I sometimes have problems with algae after setting up a tank with garden soil, because considerable iron is released into the water during the first two months (see page 131). Only after the soil has ‘settled down’, does the iron release stop and algal problems dimminish.
a) Iron as the Limiting Nutrient for Algae
Iron’s limited availability in oxygenated water sets iron apart from all other plant nutrients.3 This is because free iron (Fe2+ and Fe3+), which is the only form that algae can use [28], doesn’t ordinarily accumulate in the water. It either forms various iron precipitates (FeOOH, FeCO3, etc) or binds to dissolved organic carbon (DOC).
It is not surprising that most natural freshwaters contain only small amounts of iron, most of it bound to DOC. Indeed, the iron concentration of most oxygenated surface waters is less than 0.2 mg/l, and almost none is in the free form that algae (or plants) can use [26]. Hardwater lakes, in particular, may have little available iron. Thus, one investigator [29] found algal growth to be limited by iron in several natural lakes. For example, phytoplankton cultures from Lake Tahoe (U.S.) were greatly stimulated by adding as little as 0.005 ppm Fe.
Enormous areas of open ocean have limited algal growth despite relatively high nitrate and phosphate levels. Because these areas are far removed from terrestrial sources of iron (e.g., soil dust), iron is present in exceedingly small amounts, less than 0.000056 ppm. Thus, when investigators added iron to experimental bottles containing these algae and their natural ocean water, algal growth was stimulated [30].
My point is that because iron doesn’t stay around very long in oxygenated water, it can limit algal growth—in aquariums as well as oceans. Unlike phosphate and other plant nutrients, which can and often do accumulate in aquarium water, the reservoir of free iron in aquarium water is limited.
Plants can get their iron from the substrate, but algae depend on free iron (Fe2+ and Fe3+) in the water. Although iron in the water is indeed bound up, often to dissolved organic carbon, it is made transiently available by a common process called the ‘photoreduction of iron’. The reaction for the photoreduction of DOC-bound iron is:
DOC-Fe3+ + light ⇒ Fe2+ + oxidized DOC
This light-requiring reaction, which also applies to manganese and copper, is greatly accelerated by DOC [31,32,33]. The photoreduction of DOC-bound iron is invariably accompanied by the decomposition of DOC (see page 59).4 The Fe2+ released may be taken up by algae or quickly oxidized to Fe3+, which can also be taken up by algae or bind to fresh DOC, whereby the process repeats itself.
Different investigators demonstrated iron photoreduction using various fluorescent light sources (‘Cool-white’, ‘Daylight’, Vita-Lite™), and sunlight. UV and blue light induce the most photoreduction, because only wavelengths below about 500 nm are energetic enough to break the chemical bonds [31].5 Thus, investigators showed that only wavelengths below 520 nm released free iron from one DOC-chelated iron (Figure X-1). Algae grew well under normal light with chelated iron as the only iron source, but when light wavelengths below 520 nm were filtered out, the same algae became iron deficient and would not grow.
Figure X-1. Algal Growth and Iron Photoreduction.
‘Growth’ was determined from chlorophyll a fluorescence.
‘Normal Light’ cultures of algae were grown in nutrient media under continuous ‘Cool-white’ lighting at 120 µmol/m2/s. The only Fe source was ‘HN’, a hexanuclear Fe and sorbitol complex, which is a type of DOC-bound Fe.
‘Restricted Light’ cultures were grown under identical conditions except light wavelengths below 520 nm were filtered out.
As a control, investigators showed that the algae could grow with restricted light if adequate Fe was present. Thus, the non-growth of the ‘Restricted light’ culture in Fig. X-1 was due to Fe limitation, not light limitation.
{Figure from Rich [18] redrawn and used with the permission of the American Society of Limnology and Oceanography.}
Iron binds to a variety of chemicals and different types of DOC. These iron complexes all have their own peculiar ‘iron-binding tightness’ and susceptibility to both photoreduction and chemical reduction [34]. Thus, algae may, indeed, have access to some iron even in the dark. However, algae will get a far larger supply in the presence of light and DOC. Thus, Fe2+ levels in one lake were found to be almost 5 fold higher at midday when light intensity was greatest than at night [31]. In natural systems (and aquariums) the photoreduction of DOC-bound iron is probably essential to supplying algae with iron.6
Aquatic plants readily take up iron directly from the water [35], even when planted in iron-containing substrates [36,37]. For example, iron uptake by Hydrilla planted in a peat substrate was shown to actually equal iron precipitation as a means of removing iron from oxygenated water [36]. Plants would continuously drain free iron (Fe2+ and Fe3+) from aquarium water, thereby depriving algae of a much-needed nutrient.
Q. My tank has a soil underlayer, CO2 injection and intense light. After I added iron, Fe went from 0.25 ppm to 0.03 ppm in 48 hr. Is 0.03 ppm Fe enough to make algae starve?
A. No. Algal growth can be stimulated by as little as 0.005 ppm, which you probably can’t measure.
The fact that Fe is rapidly removed from the water does not mean that you need to add iron. Plants quickly take up Fe from the water, even though the substrate can provide the iron they need. Moreover, plants will take up much more Fe than what they need (see page 18 ‘Metal Uptake in Plants’). Therefore, I would not feed your plants Fe based on water iron levels.
High-tech tanks with CO2 injection, probably require some iron and/or micronutrient fertilization of the water. However, I would use these fertilizers sparingly and only if the plants show symptoms of iron deficiency (interveinal chlorosis of younger leaves).
Let your plants drain the water of iron. It’s one way plants can deprive algae of an important nutrient, and thus, compete with them more effectively.
In aquariums containing soil underlayers, fertilization with chelated iron is almost surely unnecessary. Soils have enormous quantities of iron (see page 83). Not only do they contain plentiful iron, but also the anaerobic conditions that keep some iron in the free, unbound form that plants can use.
In my opinion, the substrate– not the water– is the best place to provide plants with iron. Recommendations to maintain a certain water level of iron may be based on work that does not apply to the home aquarium. For example, aquatic botanists and hydroponic growers routinely add EDTA-chelated iron, but their plants may be sterilized beforehand or grown emergent. Under these circumstances, chelated iron is essential and will not promote algae. But what works for aquatic botanists and hydroponic growers may not work for the home aquarist.
Hobbyist’s Comment. I thought you might be interested in my experience with controlling algae by limiting iron in the water. The tank is a 55 gal with 32 Tetra-size fish, heavily planted, CO2 injection, KH of 4, phosphates = 0, nitrate = 10 or less, 110 watts of light, and gravel over a no longer used Undergravel Filter.
I was having severe problems with thread algae as well as some red algae. At the same time, my large Amazon swordplant almost died. My aquarium dealer suggested fertilizing with chelated iron, so that the water Fe levels never falls below 0.1 ppm. Soon I had a beautiful Amazon sword, but the thread algae increased greatly. I could no longer control the algae by hand removal.
Eventually, I realized that the fact that the substrate was not iron-enriched might be the problem. So, I limited iron additions at water changes to once every two weeks. At the end of the week, water Fe was at or near zero, so half the time my plants had no free water iron. I added some laterite balls around plants and potted a couple of Crypts in laterite and potting soil.
The results were amazing. I noticed less algae almost immediately. Two months later thread algae is zero, algae on glass is 5-10% of former infestation, and I can find only two leaves on all my plants with a couple of tufts of red algae.
3.Allelopathy
If iron limitation was the only factor controlling algal growth in my aquariums, then the iron-rich lava rocks in my tanks should be covered with algae. They are not. Allelopathy may be the ‘wild-card’ in the formula for controlling algae. Different species of aquatic plants produce different allelochemicals. Ditto for algae. Thus, the possiblities for unpredictable interactions in the home aquarium are truly enormous.
However, all aquatic plants contain chemicals that are mildly inhibitory to algae (see page 41). Allelopathy may explain the unexplainable, why algae, which has so many advantages, is unable to take over heavily planted aquaria– even when nutrients and light are abundant.
Conversely, some algae secrete allelochemicals that inhibit plants (see page 49). Thus, the hobbyists should be aware that if algal growth becomes excessive in the aquarium, these allelochemicals may inhibit plant growth. The hobbyist can easily remove the algal allelochemicals by water changes and adding charcoal to the filter.7
D.Intensive Care for Algal Takeovers
In every home aquarium, there’s a delicate balance between plants and algae. Occasionally, even in aquariums that are set up for ideal plant growth and that have never had problems, algae may seemingly arise from nowhere and take over the tank. These takeovers may defy explanation or standard treatments.
For years I had no problems with algae, even though I had added other hobbyist’s most troublesome algae to my tanks to test some of my ideas about algae control. Eventually, though, two types of algae (‘green water’ and ‘green mat’ algae) became troublesome in some of my tanks. I found that these algae were not going away no matter how many water changes or how much hand removal I did. Thus, I devised a plan to rid the tanks of this algae using a combination of measures that would shift the balance towards the plants rather than algae.
For example, after I set up my 45 gal tank with garden soil, it developed a bad case of green water algae. The tank was unsightly, and the plants were not doing well. I measured a very high pH during the day confirming my suspicion that algal photosynthesis was driving the pH so high that the plants were being inhibited by a lack of CO2.
First, I did a complete water change to remove the majority of the algae. (Although I knew the algae would grow right back, I didn’t want a mass of dying algae to pollute the tank.) Second, I added fresh charcoal to the filter. (This would remove DOC along with its propensity to provide iron to algae much as artificial chelators do. Also, charcoal would remove any allelochemicals or toxins released by the algae that might be inhibiting the plants.)
Because the tank received intense sunlight through a window, I taped cloth across the entire back of the tank. I also taped duct tape across the bottom 3” of glass at the tank’s back to keep all strong light off the soil underlayer. (My garden soil, a Southeastern clay, contains so much iron that exposure to intense light generates soluble iron, some of which will escape into the overlying water and stimulate algae.)
I also removed one of the two 40 watt light bulbs. Now the light source for the tank was one 40 watt Cool-white bulb and some diffuse window light entering through the cloth.
Finally, I added water lettuce (Pistia stratiotes), which is a floating plant, to the tank. Even though I had removed some of the lighting, the water lettuce was still getting some good window light coming in through the top of the tank. The water lettuce immediately started growing, forming long (6-10”) bushy roots quite suited for pulling nutrients out of the water.
For the first week, the green water algae held on, visible now only as a slight cloudiness. During the second week the tank water started to clear. After two weeks the tank cleared completely.8
For hobbyists that have extra money and less patience, I would suggest using a UV sterilizing filter, a main-stay for many pond hobbyists. It is safer than adding chemicals and probably more effective than a diatom filter. The UV light will quickly kill the green water algae cells as they slowly pass through the filter. The UV sterilizer would have no detrimental effect on the biofilter, tank ecology, etc. Indeed, it can help prevent and control fish diseases (see page Error! Bookmark not defined.).
I used similar measures for an infestation of green-mat algae in a 29 gal tank. However, instead of changing water, I removed the algal mats by hand. All the other measures were identical to what I used for the 45 gal. It is hard to say which measure was responsible for tipping the balance in favor of the plants. Each one is designed to give plants a slight advantage.
Hobbyists should keep in mind that the strategy I used was designed for two green algaes that typically thrive in hardwater with fairly intense light. The strategy would probably need to be altered for infestations of red and brown algae in softwater tanks. In a softwater tank with algae problems, I would try to increase water hardness and add floating plants and fast-growing hardwater plants (e.g., Vallisneria spp.) to the tank.
Pistia stratiotes (Water Lettuce). This floating plant, which has been used in waste water treatment, is a good tool for combatting algal takeovers. It doesn’t have to compete with algae for CO2, and like all emergent plants, it has the “aerial advantage”—the capacity to grow much, much faster than submerged plants. Its rapid growth removes water nutrients like iron that stimulate algal growth. Finally, it automatically reduces light levels in the tank.
Drawing from IFAS [39].
Q. I set up a 20 gal trial tank. The substrate has 1.5” (3.8 cm) of topsoil covered with about 1” of 2-3 mm gravel. Lighting is from three 20 watt full-spectrum bulbs. The tank also gets some direct sunlight. Tank is stocked with Vallisneria, Sagittaria, Aponogeton crispus, and Saururus cernus, which are doing fairly well. The problem is that there is an algal bloom of green water that I can’t get rid of. I’ve tried everything I could find in the literature and over the Internet. Nothing has worked. Do you have any suggestions?
A. Yes. Your tank has more light than what these submerged plants can handle. I would either reduce the lighting (remove one of the fluorescent lights) or start a colony of floating plants. Floating plants will cut down on the light and remove nutrients, especially iron, from the water. Other things, you might try are:
1.Tape a piece of diffusive paper or cloth to the back of the tank to further reduce side lighting
2.Replace one of your 3 full spectrum lights with one Cool-white light
3.Add fresh charcoal to the filter
4.Run duct tape along the entire bottom/back 3” of the tank so that the soil underlayer is never exposed to sunlight
Adding floating plants is particularly effective in dealing with green water algae, especially in situations like yours where there is plenty of light.
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1The exceptions are various marine turf algae associated with tropical coral reefs that can maintain rapid growth with no apparent photoinhibition under the full tropical sun [8]. Turf algae have been used by some marine aquarists to filter their large salt-water tanks [9]. (Rapid growth of these unique algae can often reduce both nitrates and ammonia to undetectable levels.)
2 The light requirements of plants for proper development were found to be even greater. Apparently, Elodea canadensis could survive at 11 µmol/m2/s, but it required 24 µmol/m2/s for branching and more than 54 µmol/m2/s for root development. (Light required for maximum growth of E. canadensis is 290 µmol/m2/s [15].)
3Manganese (Mn) is the only other plant nutrient that might not accumulate in aquarium water, because like Fe, it forms insoluble oxides. However, Mn is more soluble than Fe, and algae and plants require considerably less Mn than Fe [26,27]. Therefore, Mn has less potential to limit algal growth.
4The reaction also applies to artificially chelated iron such as FeEDTA, where EDTA is oxidized and decomposed as it releases Fe2+.
5The 280 to 400 nm portion of the light energy spectrum encompasses UV (ultraviolet) light, while the 400 to 500 nm range consists of violet and blue light (see Fig XI-2 on page 181).
6 Iron is also released by acidity and anaerobic conditions, which do not require light and would occur mainly in aquarium substrates. However, in the aquarium water where there is light, oxygen, and DOC, photoreduction would be the main mechanism for releasing iron.
7Activated carbon (e.g., aquarium ‘charcoal’) is used by municipal treatment plants to remove organic chemicals from water by the non-specific process of adsorption. Although virtually any organic chemical would be removed, specific compounds on a list of 56 organics reported to be absorbed are: aldrin, diquat, gasoline, lindane, malathion, paraquat, phenols, PCB, rotenone, and simazine [38]. In aquariums it would remove almost all allelochemicals, humic substances, artificial chelators, antibiotics, and dyes.
8 Green water algae is not always a curse. For example, I set up a 10 gal “Demo” planted tank at work with a female and male Betta. The tank almost immediately developed an embarrassing case of green water algae. I struggled mightily to get rid of the green water algae– to no avail. Then, my co-workers pointed out that the Bettas in the tank had spawned and there were babies! After that, I left the tank alone. (I did remove the parents who would eventually have eaten the babies.) Thirteen babies survived. They grew very well, feasting on the many little protozoa that were feeding on the green water algae and soil-associated bacteria in this tank’s rich ecosystem.