Chapter II.
PLANTS AS WATER PURIFIERS
Aquatic plants protect fish from toxic ammonia, nitrite, and heavy metals. Intrinsic to the idea of plants as water purifiers are three facts:
1.Aquatic plants readily take up heavy metals
2.Humic substances from decomposed plant tissue detoxify heavy metals
3.Aquatic plants readily take up ammonia and nitrites
A.Heavy Metals
‘Heavy metals’ are toxic to all organisms, whether they are required micronutrients (zinc, copper, iron, manganese, nickel) or environmental pollutants (aluminum, lead, mercury, cadmium, etc).1 Table II-1, which ranks several heavy metals according to their molar toxicity to various organisms, shows that mercury and copper are the most toxic.
Table II-1. Toxicity of Heavy Metals to Organisms [3].
| Organism | High Toxicity ⇒ Low Toxicity |
| Algae | Hg > Cu > Cd > Fe > Cr > Zn > Co > Mn |
| Fungi | Hg > Cu > Cd > Cr > Ni > Pb > Co > Zn > Fe |
| Fish | Hg > Cu > Pb > Cd > Al > Zn > Ni > Cr > Co > Mn |
| Flowering Plants | Hg > Pb > Cu > Cd > Cr > Ni > Zn |
Abbreviations: Al = aluminum; Cd = cadmium; Co = cobalt; Cr = chromium; Cu = copper; Fe = iron; Hg = mercury; Mn = manganese; Ni = nicklel; Pb = lead; and Zn = zinc.
1.Metals in Our Water Supplies
Which heavy metals in tapwater might be a problem for our fish? If human standards were the same as fish standards, water good enough for human drinking would be good enough for fish. However, this is not the case, especially for zinc and copper. First, fish standards are much higher than those for humans (Table II-2). For example, fish require that Cu levels be 65 times lower (0.02 ppm versus 1.3 ppm) and Zn levels 50 times lower (0.1 ppm versus 5.0 ppm). Second, Cu and Zn are considered to be non-toxic to humans. Their standards are set for aesthetic reasons (taste, porcelain staining, etc) and are not federally enforced. This means that drinking water could conceivably contain enough copper and/or zinc to harm fish.
Table II-2. Some Heavy Metal Standards for Humans and Fish [4,5].
| Metal | Humans (ppm) | Fish (ppm) |
| Cadmium | 0.005 | 0.01 |
| Chromium | 0.1 | 0.05 |
| Copper | 1.3 | 0.02 |
| Lead | 0.015 | 0.1 |
| Mercury | 0.002 | 0.01 |
| Zinc | 5.0 | 0.1 |
Q. I am concerned with your conclusion regarding the extent of metal contamination in aquariums. It is unlikely that most municipal water systems would contain enough metals to seriously harm aquatic life; the only other source of metal contamination is from pipes.
A. I am not convinced. Hobbyists blithely add copper to their tanks to control algae and parasites with no idea of how toxic copper can be. Both zinc and copper could be in drinking water at levels that could be toxic to fish. My own well water has enough zinc to kill invertebrates in my aquariums and keep brine shrimp eggs from hatching (see page 183). A few hobbyists have reported problems from excessive copper in municipal tapwater. Other hobbyists might not even recognize problems from metal toxicity. (Sick fish, poor hatches from brine shrimp eggs, dead invertebrates, and plant meltdowns are so easily attributed to other causes.)
Metal toxicity has rarely been discussed in the aquarium literature. This interesting topic, which is related to micronutrient nutrition in plants, fish physiology, and decompositional processes in aquariums, deserves some attention.
Municipal water treatment procedures such as coagulation-flocculation and lime softening help remove Zn and Cu. Thus, metal contamination of city water would seem unlikely. However, high copper levels have been reported in certain areas. For example, several Connecticut towns (Bridgeport, Hawkstone, Norfolk, etc) in 1997 reported ‘high-risk’ areas with Cu levels ranging from 0.14 to 1.1 ppm. And one hobbyists from Massachusettes has reported aquarium problems arising from Cu levels in the city water that fluctuate from 0.5 to as high as 2 ppm.
Q. Surely, if the water is safe for humans to drink, it must be okay for the fish?
A. We humans don’t live and breathe in water, so our dosage is small. Furthermore, much of the metals that enter our digestive tract would be inactivated by binding to organic matter (partially digested food).
In contrast, fish gills and skin are directly exposed to whatever metals are in the water. Heavy metals ‘sneak in’ through pathways designed for nutrient uptake, particularly calcium. Thus, in metal-contaminated water, the fish will contain high metal levels-- and be injured accordingly.
Ground water, especially water from private wells, could also contain harmful levels of zinc and copper. Indeed, one survey [6] of U.S. ground water shows huge variations in both Cu (0.01 to 2.8 ppm) and Zn (0.1 to 240 ppm). Additional heavy metal contamination of drinking water may come from the leaching of metal pipes, heating coils, and storage tanks.
2.Mechanisms of Heavy Metal Toxicity
Many metals are toxic, because they capriciously bind to organic molecules within organisms. For example, mercury binds to the sulphydryl groups (-SH) found on virtually all proteins, thereby inactivating the proteins and their cellular functions.
Iron toxicity occurs in plants as well as humans (e.g., hemophiliac patients overloaded with iron from continuous blood transfusions [7,8]). The toxicity occurs when cellular oxidation of iron (Fe2+) produces highly reactive oxygen radicals, which can kill cells by destroying DNA, membrane lipids, and proteins.
However, the most common mechanism of metal toxicity is when a foreign metal displaces another metal from its specific binding site on organic molecules. For example, nickel can displace zinc from its proper binding site on the enzyme carbonic anhydrase thereby inactivating the enzyme [1]. (Many enzymes require the attachment of a specific metal in order to function.)
Heavy metal substitution for calcium is often an underlying factor in metal toxicity. All cell membranes have a phospholipid bilayer that is stabilized by Ca. Intruding heavy metals can displace the desired Ca and disrupt cell membrane structure and function [1]. And calcium’s unique role as a secondary messenger in cells insures that many functions of almost any organism are susceptible to metal toxicity [9,10].
3.Metal Toxicity in Fish
While high levels of heavy metals can cause gross tissue damage and death in fish [11], the most common effects (behavioral changes and reproductive failure) are from minor contamination. Behavioral changes result when heavy metals disrupt the release of neurotransmitters and hormones from producing cells [12].
Fish had problems capturing live daphnia following a 4 week exposure to lead (Table II-3). Control (untreated) fish reacted to the daphnia much further away than Pb-exposed fish. Also, lead accumulated in the brains of Pb-exposed fish.
Table II-3. Effect of Lead (Pb) on Feeding in Minnows [17].
| Variable | Controls (no lead) | Lead Exposure | |
| 0.5 mg/l | 1.0 mg/l | ||
| Reaction distance (cm) | 2.7 | 1.9 | 1.7 |
| Miscues during feeding (number of) | 9.0 | 50 | 49 |
| Time to consume 20 daphnia (min) | 1.4 | 6.2 | 5.5 |
| Pb in fish brain (mg/l) | Not detected | 0.45 | 0.82 |
Low levels of heavy metals may affect normal fish behavior such as schooling, feeding, swimming, and successful spawning. For example, copper was shown to significantly reduce the swimming performance of rainbow trout [13]. Continuous exposure to aluminum decreased the appetite and growth rate of young trout [14]. Lead had no effect on the growth of young male trout, but it profoundly affected sperm production [15]. Copper was shown to impair the smell receptors of salmon, which are critical to its spawning migration [16].
Fish are guided by their own unique circadian rhythms, which are controlled by neurotransmitter levels within specific regions of the fish brain. By disrupting neurotransmitter function, heavy metals can affect the natural circadian rhythm of fish [12]. For example, when sea catfish were exposed to 0.1 ppm copper, they lost their normal circadian rhythm and became hyperactive (Fig. II-1). That is, treated catfish were more active both day and night, whereas untreated (control) catfish were much less active during the day, especially in the afternoon.
Figure II-1. Diel Activity of Sea Catfish as Affected by Copper. Sea catfish were exposed to 0.1 mg/l of Cu for 3 days and then monitored for activity over a 24 h measurement period. Activity was determined when fish tripped a photodiode as they moved between compartments.
(Fig. 1 from Steele [18] redrawn and used with kind permission from Kluwer Academic Publishers.)
Fish are most sensitive to heavy metals during their developmental stages. Thus, while a particular metal concentration might be safe for adult fish, it could injure fish during a critical development phase. For example, the yolk sac membrane (chorion) was very fragile and easily ruptured in embryos exposed to just 0.3 ppm of zinc [19].
Table II-4 shows standards for seven heavy metals on various freshwater fish. These standards, which are based on the sensitivity of developing fish, are much more stringent than the general standards listed earlier in Table II-2.
Table II-4 Heavy Metal Standards for Sensitive Life Stages of Fish [19].
| Metal | Fish | Metal’s Effect on: | Maximum Acceptable Concentration (ppm or mg/l) |
| Cadmium | Flagfish | Spawning | 0.004- 0.008 |
| " | " | Juvenile mortality | 0.003- 0.017 |
| Copper | Brook Trout | Juvenile mortality | 0.010- 0.017 |
| Chromium | Brook Trout | Juvenile mortality | 0.20- 0.35 |
| Lead | Brook Trout | Juvenile deformity | 0.058- 0.12 |
| Mercury | Fathead Minnow | Juvenile growth | < 0.00026 |
| Nickel | Fathead Minnow | Egg hatching | 0.38- 0.73 |
| Zinc | Flagfish | Growth | 0.026- 1.2 |
| " | Fathead Minnow | Egg fragility | 0.078- 0.15 |
4.Metal Toxicity in Plants
Plants afflicted with metal toxicity exhibit various symptoms that might be interpreted incorrectly as nutrient deficiencies. Symptoms of aluminum toxicity for Vallisneria are premature browning and senescence of leaf tips [22]. Excesses of either copper, manganese, or zinc can induce iron deficiency and chlorosis [23].
Iron toxicity has been studied in at least two aquatic plant species. Investigators [24] reported a 75% growth reduction in the pondweed Potamogeton pectinatus after adding iron (1.2 mg FeCl3/g) to the substrate. Leaves turned brown. Roots turned pale or red brown in color, and root growth was stunted. Hydrilla verticillata, exposed to well water containing 1.2 ppm Fe, became covered with a rusty brown color and began to decay [25].
Q. I added iron (as FeCl3) to my tank to reduce phosphates in the water. (Phosphate reacts with iron to form insoluble iron phosphate.) Six days afterwards, the phosphate concentration had decreased from 0.6 ppm to 0.1 ppm, but I began to see phosphate deficiency in some of my plants. It started with the slower growing plants. For example, the Cryptocoryne had brown spots on their leaves, which expanded until the whole leaf was affected. Fast-growing plants species seemed unaffected by the P deficiency, which surprised me, as these plants usually require more nutrients.
A. I think you’re confusing phosphate deficiency with iron toxicity. Phosphate levels of 0.1 ppm in the water are more than sufficient for plant growth. The brown spotting of the leaves suggests iron toxicity. The browning is due to iron deposits in the leaves, as the plant tries to store the excessive iron coming in.
The fact that your faster growing plants did not show the ‘deficiency’ supports my contention that the problem is metal toxicity not nutrient deficiency. Metal toxicity in plants can be overcome by rapid growth. Faster growing plants ‘dilute out’ the problem; metal concentrations within the tissues decreases with new growth. Slow-growing plants are at a disadvantage; the metal concentration within the plant builds up to injurious levels.
5.Factors that Moderate Metal Toxicity
Because metal toxicity is so often affected by other factors, it is very difficult to say that a particular metal concentration is toxic. It may or may not be depending on water hardness, pH, organic matter, and the target species. In general, metal toxicity is reduced when metals are bound to organic matter, soil particles, or carbonate ions. These bound metals are less likely to be absorbed by plants and fish.
In general, metal toxicity is a much greater problem in soft, acidic water. Many scientific studies were prompted by environmentalist’s concerns over the acidification of natural lakes by acid rain. As lakes acidifies to pHs below 5.5, heavy metals like aluminum, copper and zinc are released from the sediment into the water.
Experiments show that water hardness (see page 86) by itself influences metal toxicity. Thus, trout exposed to 1.5 ppm of aluminum had a 45% mortality in softwater but only 10% in hardwater [14]. Daphnia exposed to 0.13 mg/l of zinc survived less than 10 days in softwater but over 50 days in medium hardwater [26].
Copper toxicity to fish may be lowered significantly (up to 90%) in hardwater, due solely to the competition between copper (Cu2+) and calcium (Ca2+) for fish uptake [27]2. Investigators [9] showed that if they increased water calcium from 4.4 to 43 ppm, heavy metal uptake (and toxicity) in mussels was greatly reduced. Ca was found to be much more important than Mg in preventing metal uptake. The investigators hypothesized that calcium’s competition with heavy metals for uptake via the calcium channels of cells was the main mechanism for hardwater’s protective effect.
pH mildly influences metal toxicity, with neutral pH providing the most protection. Thus, copper was two times more toxic to rainbow trout after the pH was lowered from pH 7.2 to pH 5.4 [27]. Aluminum is especially influenced by pH; it is only toxic at extremely acidic pH (< 5.5) or alkaline pH (>8) [14]. In general, metals will be more toxic in soft, acidic water and less toxic in hard, alkaline water.
Q. I’m using an aluminum reflector that may drip some aluminum condensate into the water. Should I be concerned about aluminum toxicity?
A. No. If your aquarium water is between pH 6 and 8, aluminum is not toxic.
Although water hardness and pH can individually reduce metal toxicity, organic carbon confers the greatest protection by far [14]. Thus, for metal toxicity in flagfish, investigators [31] showed that organic carbon provided 27 times more protection than water hardness.
Dissolved organic carbon (DOC) is found in lakes and rivers at concentrations ranging from 1 to 30 mg/l (average is 6 mg/l) [32]. DOC is often invisible except for a yellowish color or the soapy foam it forms in flowing stream waters (and aquarium protein skimmers).
Metals readily bind to DOC. Every mg of DOC has the capacity to bind 1 µeq of metal [33].3 Bound metals are not readily taken up, and therefore, are much less toxic than soluble metals [34].4 Examples of DOC that bind metals are: amino acids, organic acids (e.g., citric acid), polypeptides, proteins, and humic substances.5 Fig. II-2 shows how 3 organic compounds (glycine and two humic compounds) bind copper (Cu).
Fig. II-2. Examples of Copper (*Cu) Binding to Organic Carbon. Figs. 11.28 and 11.29 from Thurman [33] used with kind permission from Kluwer Academic Publishers.
Humic substances bind to heavy metals more tightly than calcium [9]. This means that humic substances will alleviate metal toxicity, even if the water is soft and contains little Ca. Several studies have shown that either DOC (or its humic acid component) decrease metal toxicity. For example, when natural DOC was removed from lake water by charcoal filtration, copper toxicity (4 day LC50) to minnows increased over ten-fold [36]. In another study, most daphnia were killed within 24 to 48 hr by 0.015 ppm copper, but when 1.5 ppm humic acid was added, they survived at least 40 days [37]. Rainbow trout continuously exposed for 16 days to 0.1 ppm of soluble aluminum had no deaths and grew about 40% faster in the presence of humic acid [14].
Q. The yellow color of ‘aged aquarium water’ represents a polluted, unhealthy condition for fish. Therefore, the water in aquariums should be changed frequently.
A. Not necessarily. In an established aquarium containing plants, the yellowish color of aged aquarium water is from humic substances not from raw animal waste. Humic substances are formed as plant matter decays.
Hobbyists have debated the value of this old, yellowish water for years, with some saying that ‘aged aquarium water’ represents an unhealthy environment for fish. In the case of heavy metals, scientific evidence suggests otherwise. The color is due to humic substances, which bind and chelate heavy metals and reduce their toxicity to fish. And even if the aquarium water is not colored, humic substances will probably be there.
Humic substances derived from plants help protect fish from metal toxicity.
Investigators [31] studied DOC’s effect on toxic mixtures of aluminum, zinc, and copper towards flagfish in soft, acidic waters. (Note: these particular metals often increase when lakes acidify.) Fish mortality from the metal mixture was reduced 2 to 15 fold by lakewater DOC. The investigators concluded that young flagfish probably could not survive in acidified, softwater containing less than 2.2 mg/l of total organic carbon.
Metal binding to DOC (or its humic substance component) prevents metals from being taken up by organisms. This is true for plants as well as fish. One investigator showed that the water hyacinth didn’t take up copper (Cu) when humic acid was present (Table II-5). The plant removed 94 % of the copper from a 1 mg/l solution of copper with no humic acid. Some of this copper (0.94 mg) was found in the plant’s tissue. In the humic acid solution, though, copper was not removed from the water and no copper was found in the plants. This is because the copper was bound to the humic acids and could not be taken up by the water hyacinth.
Table II-5. Effect of Humic Acid on Copper Uptake by Water Hyacinth [38]. Plants were grown in nutrient media containing Cu (1 mg/l) for 1-2 weeks with or without 20 ppm of humic acid.
| Treatment | Cu Remaining in Solution (mg/l) | Cu Accumulation in Plants (total mg) |
| Control (no humic acid) | 0.063 | 0.94 |
| Plus Humic Acid | 1 | 0 |
Artificial chelators bind tightly to heavy metals. Unlike DOC, they bind metals in a one-to-one molar ratio, with a well-known order of priority [39]. For example, every molecule of EDTA was shown to bind one copper molecule in a highly predictable manner [38].
Table II-6 shows the stability constants for the formation of some EDTA metal complexes. They are listed in order of increasing ‘binding tightness’, with ferric iron the most tightly bound and magnesium the least tightly bound. Fortunately, EDTA binds much more tightly to heavy metals like Zn and Fe than Ca and Mg. For example, EDTA binds to Zn 790,000 times more tightly than it does to Ca.6
Table II-6. Stability of Metal-EDTA Complexes [40]. (Note: Although the copper-EDTA complex is not listed here, it has about the same stability as ZnEDTA [41].)
| Reaction | Log K |
| Mg2+ + EDTA4- ⇒ MgEDTA2- | 9.99 |
| Ca2+ + EDTA4- ⇒ CaEDTA2- | 11.9 |
| Mn2+ + EDTA4- ⇒ MnEDTA2- | 15.3 |
| Zn2+ + EDTA4- ⇒ ZnEDTA2- | 17.8 |
| Fe3+ + EDTA4- ⇒ FeEDTA- | 27.0 |
Metals can switch places on the EDTA molecule [41]. (That is, Ca can be ‘bumped off’ the EDTA molecule by Zn, because Zn binds more tightly to EDTA than Ca.). Thus, even though EDTA does bind to Ca and Mg, it will still alleviate metal toxicity in hardwater.
Q. Will chelated iron fertilizer (FeEDTA) reduce metal toxicity to fish?
A. No. This is because the EDTA is already bound to a metal, in this case iron (Fe). Since iron is the metal that binds most tightly to EDTA, metals like zinc or copper are not going to exchange for the iron in the FeEDTA. Only if you add pure EDTA will zinc, copper, and other toxic metals be removed. (Commercial water conditioners for aquariums often contain EDTA.)
Q. I don’t understand. Many plant-growers use chelated iron as a fertilizer. If iron binds so tightly to EDTA, how can chelated iron provide iron to plants?
A. Iron is slowly released (as Fe2+) from FeEDTA in the presence of light (see page 167). This process, which also applies to DOC-bound iron, provides iron to plants.
Species variation in response to metal toxicity is genetically fixed; species that are more sensitive to metal toxicity don’t easily become resistant. For example, one strain of terrestrial grass eventually adapted to lead-contaminated soil, but it took about 100 years [42].
One way plants protect themselves is by producing their own metal chelators [43]. For example, an aluminum-resistant strain of wheat was found to release more of the chelator malate from its root tips than an aluminum-sensitive strain when exposed to increasing amounts of Al [44].
Plant and fish species that developed in hard alkaline waters during their evolution had little exposure to heavy metals. As a consequence, these organisms have not developed the physiological mechanisms that would protect them from metal toxicity.
Growth, by itself, may reduce or eliminate metal toxicity by simply diluting out the metal’s concentration within the organism’s tissue. For example, investigators showed that they could eliminate aluminum and iron toxicity in Vallisneria americana by fertilizing the plants with CO2 [45]. Enhanced plant growth diluted the aluminum concentration within the plant from 2,000 ppm to 693 ppm.
Soil particles readily bind heavy metals (see pages 125-127). Investigators [46] analyzing heavy metal association with soil particles in two South Carolina streams found that lead (Pb) was strongly associated with soil particles, especially clay.
Comment. For a long time, I had trouble keeping Rainbows and Tanganyikan cichlids. I would do a water change, and these fish would inexplicably die, while the Tetras were unaffected. I had very poor luck raising fry of most sorts in my water. In addition, I had trouble keeping ‘beginner’ plants like Vallisneria, Hornwort, and Sagittaria, but had no problem with Cryptocoryne. Later I learned that my city water sometimes contained as much as 2 ppm of copper.
Reply. The fish and plants injured by the copper in your tapwater originate from hardwater. They would be expected to more sensitive to heavy metals than the Tetras and Cryptocoryne, which originate from soft, acidic waters.
6.Metal Uptake by Plants
Aquatic plants readily take up heavy metals. Figure II-3 shows that Elodea nuttallii rapidly took up copper and zinc. Metal uptake by roots was especially rapid. Within 2 hours, roots exposed to 3.2 ppm zinc had accumulated over 1,000 mg/kg of zinc, while leaves had accumulated about 300 mg/kg. The waterhyacinth, which is particularly resistant to metal toxicity, was shown to remove virtually all Cu from concentrated copper solutions (1 and 10 mg/l) within 1 to 3 weeks without any apparent harm to the plants [38].
Figure II-3. Cu and Zn Uptake by Leaves and Roots of Elodea nuttallii.
Leaf or root sections were exposed to Cu or Zn (3.2 ppm) and then analyzed for metal accumulation in terms of dry wt.
{Fig. 1 from Marquenie-van der Werff [48] redrawn and used with permission of Urban & Fischer Verlag, Niederiassung Jena, Germany.}
Metal uptake may have little to do with the plant’s nutrient requirements, as plants take up metals like Pb (lead) that they do not use. Uptake of metals (required or not) increases proportionally with the water’s metal concentration [49] and can greatly exceed the amount required. For example, Hydrilla verticillata did not become iron-saturated until water levels of chelated iron reached 6 mg/l and its tissues contained over 21,000 mg/kg iron [50]. [Note: aquatic plants only require about 60 mg/kg in their tissues (see pages 104-105).]
Table II-7, documenting work with the duckweed Spirodela polyrhiza, correlates inhibitory metal concentrations with how much metal is found in the plant’s tissue. For plants grown in solutions containing 3.7 mg/l Pb, growth is inhibited 50% and the plant tissue will contain over 6,700 ppm lead.
Table II-7. Metal Uptake by Spirodela polyrhiza
[51]. The metal concentration in the growth media and in the plants associated with 50% growth inhibition (EC50) was calculated after exposing 10 plants to 5-6 different metal concentrations for 4 days.
| Metal | Metal Concentration Correlated with Growth Inhibition | |
| In Media (mg/l) | In Plant Tissue (mg/kg) | |
| Cadmium | 0.089 | 773 |
| Cobalt | 0.14 | 590 |
| Chromium | 0.37 | 156 |
| Copper | 0.11 | 502 |
| Nickle | 0.11 | 1,290 |
| Lead | 3.7 | 6,730 |
| Zinc | 0.93 | 3,510 |
Giant Duckweed (Spirodela polyrhiza). S. polyrhiza, like many other aquatic plants, can rapidly remove large quantities of heavy metals from contaminated water (see Table II-7). Plants are about 3 times bigger than ordinary duckweed (Lemna minor).
Plant drawing from the IFAS [52].
B. Ammonia
Ammonia is one of the most important and common pollutants of aquariums. Fish and bacteria excrete ammonia as a waste product of their metabolism. Ammonia (NH3), which is toxic, exists in equilibrium with non-toxic ammonium (NH4+) in the following reaction:
NH4+ + OH- ⇔ NH4OH ⇔ NH3 + H2O
The percentage of ammonia in a solution with a given N concentration changes dramatically with pH. Typically, there is a 10 fold increase in ammonia for every 1 unit increase in pH as NH4+ converts to NH3 in the above equilibrium reaction. For example, if the pH increases from 7.0 to 8.0, the % of N that is NH3 increases from about 0.33% to 3.3%, while the % of N that is NH4+ correspondingly falls from 99.7% to 96.7% [53]. Thus, the higher the pH, the greater the NH3 concentration and toxicity of a given concentration of inorganic nitrogen.
1.Ammonia Toxicity in Fish
Fish differ in susceptibility to ammonia. For example, lethal ammonia concentrations for rainbow trout were found to range from 0.2 to 1.1 mg/l of NH3, while those for channel catfish were between 1.8 to 3.8 mg/l of NH3 [54].
Chronic ammonia toxicity impairs reproduction (e.g., delays spawning and reduces egg viability). Long-term (1 wk to 3 mo.) exposure to ammonia concentrations as low as 0.002 to 0.15 mg/l of NH3 can suppress appetite and inhibit growth of young fish [54]. Other symptoms may be ragged fins or deformities in young fish such as missing gill covers, or the fish may simply become increasingly susceptible to disease.
Recommendations for safe ammonia levels vary. Water quality experts recommend that ammonia (NH3) levels be kept below 0.01 mg/l in natural freshwaters to avoid chronic effects [55]. Aquarium hobbyists, who measure total ammonia (NH3 plus NH4+) with their test kits, should keep total ammonia below 0.02 mg/l for their freshwater fish [56].
2.Ammonia Toxicity in Plants
Ammonia can kill plants or reduce their growth [57]. Aquatic plants vary in their ability to tolerate ammonia– even within the same genus. For example, Elodea canadensis showed a slight (~20 %) reduction in photosynthesis when exposed for 7 days at pH 8.4 to 3.2 mg/l ammonium.7 In contrast, both Elodea nuttallii and E. ernstae were either unaffected or stimulated by 9.6 mg/l NH4+ [58].
Other studies show that Potamogeton densus growth was inhibited by 5.0 mg/l NH4+, while Stratiotes aloides showed decay and destruction of plant tissue when exposed for 10 weeks to only 0.9 mg/l NH4+ [59]. High concentrations (2.6 to 26 mg/l NH4+) did not inhibit Salvinia molesta, and in some instances, stimulated growth [60].
Thus, it appears that sensitive species of aquatic plants would be harmed by about 1 mg/l NH4+. However, less sensitive aquatic plants, particularly those adapted to nutrient-rich waters, would not be harmed by concentrations as high as 26 mg/l NH4+.
Q. Is there any evidence that plants in the aquarium take up ammonia (NH3)?
A. The truth is plants cannot keep ammonia out. For ammonia diffuses freely across the cell membranes of all organisms (plants, fish, etc) [62]. Because NH3 is a gas without an electrical charge, it diffuses freely across the lipid bilayer of cell membranes. In contrast, ammonium (NH4+) has an electrical charge, and therefore, cannot cross the lipid bilayer without help (membrane transporters, enzymes, etc).
While plant uptake of charged molecules requires more work, charged molecules are also less toxic, because plants can regulate their uptake. Indeed, the toxicity of small, uncharged molecules like NH3, HNO2, and H2S may be due, in part, to the fact that cells cannot regulate their uptake and prevent them from entering the cell. Thus, all organisms are vulnerable to high concentrations of these molecules.
I would assume that plants constantly remove ammonia from aquarium water.
Plants rapidly detoxify ammonia [61]. As NH3 enters plant cells by diffusing across the cell membrane, the plant combines it with a hydrogen ion (H+) and converts it to non-toxic ammonium (NH4+) [62]. This NH4+ can be stored in cell vacuoles. Indeed, the vacuoles of Nitella clavata were found to contain over 2,400 mg/l NH4+ [64].
Plants can also detoxify ammonia by immediately using it to synthesize proteins. NH3 is combined with stored carbohydrates to form ordinary amino acids (see page 111) that make up the plant’s proteins. Plants that contain plentiful stored carbohydrates can tolerate ammonia better than carbohydrate-depleted plants.
3.Ammonia Uptake by Aquatic Plants
Most aquatic plants studied, when presented with a choice between ammonium and nitrates as their nitrogen source, take up ammonium exclusively. Only when ammonium is unavailable, do plants take up nitrates (see pages 107-108).
C. Nitrites
Problems with nitrites (NO2-) are less discussed in the aquarium hobby than those with ammonia. However, nitrites can sometimes be a problem in freshwater aquariums.
Because several bacterial processes produce nitrites (see pages 65-66), instances of nitrite accumulation are not uncommon. Nitrite levels as high as 100 mg/l NO2- have been reported in contaminated natural waters [54].8
1.Nitrite Toxicity
Oxygen is transported within blood by hemoglobin. Nitrite converts hemoglobin to methemoglobin, which is a brown-colored molecule that cannot bind oxygen. Fish hemoglobin may convert to methemoglobin when the water contains as little as 0.05 mg/l of nitrites [54].
Nitrites affect fish species differently. Thus, lethal concentrations range from 0.1 to 0.4 mg/l NO2-N for Rainbow trout to 1.6 mg/l for Mosquito fish and 10 mg/l for Channel catfish [54]. These are 3 day LC50s, which means that half of the fish were killed within 3 days. As with all toxins lower concentrations may not kill the fish outright, but they may stress the fish such that eventually they succumb to disease or other problems. For example, trout exposed to low NO2-N concentrations (0.015 to 0.060 mg/l) for 6 months showed temporary but not permanent gill damage [54].
Q. Why do brown streaks develop in goldfish when the weather turns cold?
A. Brown streaking in fins suggests nitrite poisoning. During the summer when algae and plants grow well, your fish were probably fine. In the winter, though, plant and algal growth slows, so that there is less nitrogen removal from the water. Also, in cold weather nitrification is often incomplete and nitrites tend to accumulate (see pages 65-65).
I would immediately change the water and remove debris. I would also add 1 teaspoon of ordinary table salt to each 10 gal of pond water. (The standard treatment for nitrite poisoning is to add NaCl at the rate of 20 mg/l for every 1 mg/l NO2-N [63].) I would monitor nitrite, especially during the winter months.
Nitrite is more toxic at low pH, because nitrite (NO2-) converts to nitrous acid (HNO2), which is the toxic form of nitrite [65]. Also, nitrite’s toxicity declines sharply with increasing salt (NaCl) concentration, because Cl- competes directly with NO2- for absorption by fish gills [66]. Thus, nitrite toxicity in Rainbow trout exposed to 12 mg/l NO2-N was reduced 96% by simply increasing the Cl- concentration from 1 to 41 mg/l [54]. Not surprisingly, nitrite is not toxic in saltwater [67] where the Cl- concentration is 19,000 mg/l [34].
Experimental work with the Rainbow trout [66], a fish particularly sensitive to nitrite, suggests that hobbyists should keep nitrite levels (NO2-) below 0.03 mg/l.
Nitrite is much less toxic to plants than fish. For example, investigators used media containing 14 to 56 mg/l NO2-N for their studies with nitrite uptake and assimilation in duckweed [68,69]. The relative non-toxicity of nitrites to plants is supported by work with terrestrial plants, such as one study showing that wheat seedlings were only slightly inhibited when nitrite concentrations reached 70 mg/l [70].
2.Nitrite Uptake by Plants
Although plants definitely can use nitrite as a nitrogen source, the pertinent question for aquarium hobbyists is– Do aquatic plants remove the toxic nitrite in preference to the non-toxic nitrate? No definitive answer to this question in the scientific literature is currently available.
However, when the duckweed Spirodela oligorrhiza was grown in media containing nitrate and nitrite, it clearly preferred nitrite (Fig. II-4).
Figure II-4. Nitrite and Nitrate Uptake by Spirodela oligorrhiza.
Plants that had been grown with ammonium as their sole N source were transferred to medium containing both nitrite and nitrate.
Plants were grown under sterile conditions. (Thus, bacterial processes could not have caused the observed reductions in nitrites and nitrates.)
{Fig. 4 from Ferguson [68] redrawn and used with permission of Springer-Verlag GmbH & Co. KG.}
When the same investigator grew Spirodela oligorrhiza in media containing ammonium and nitrite, it removed both ions at approximately the same rate. These results suggest that aquatic plants might remove both ammonium and nitrite equally in preference to nitrates. However, the results with Spirodela oligorrhiza can probably not be generalized to other aquatic plants. This is because nitrite uptake and assimilation into proteins requires specific transporters and enzymes, whereas ammonium uptake does not [70]. For example, the enzyme nitrite reductase required for the duckweed Lemna minor to use nitrite must be induced [69]. This induction can be blocked by ammonium suggesting that L. minor is one aquatic plant species that does not use nitrite if ammonium is available. In general, nitrite and nitrate are less desirable N sources than ammonium.
D.Using Aquatic Plants in Wastewater Treatment
Q. If aquatic plants are so good at removing toxic metals and ammonia from water, why aren’t they used more for wastewater treatment?
A. The problem is that water purification by aquatic plants requires large areas for pond sites, year-round tropical temperatures, and continuous (and often costly) plant harvesting [71].
The waterhyacinth is commonly used for wastewater treatment because of its fast growth rate. Table II-8 shows the performance of some wastewater treatment systems using the waterhyacinth. Plants were particularly effective at the Coral Springs facility where total nitrogen was reduced from 22.4 mg/l to 1.0 mg/l.
Table II-8. Effect of Waterhyacinth on the Water Quality of Wastewater [72].
BOD (Biological Oxygen Demand) is a measure of water quality. The more organic matter in the water, the more oxygen will be required (i.e., “demanded” by bacteria) in order to digest it. Unpolluted waters have a lower BOD than polluted waters.
| Location | BOD (mg/l) | Total N (mg/l) | Total P (mg/l) | |||
| Influent | Effluent | Influent | Effluent | Influent | Effluent | |
| National Space Tech. Lab, MS | 110 | 7 | 12 | 3.4 | 3.7 | 1.6 |
| Williamson Creek, TX | 46 | 6 | 7.7 | 3.3 | 7 | 5.7 |
| Coral Springs, FL | 13 | 3 | 22.4 | 1.0 | 11 | 3.6 |
Waterhyacinth (Eichhornia crassipes).
E. crassipes is one of the many floating plants that have been used in wastewater treatment. Its high growth rate, which makes it a major nuisance by blocking navigational water ways, also makes it highly effective in removing water contaminants.
While the waterhyacinth is too large for most aquariums, other floating plants more suited to aquariums (duckweed, water lettuce, water sprite, frogbit, etc) share the waterhyacinth’s enormous capacity to remove water contaminants. This is because all floating plants have the ‘aerial advantage’ (See Ch. IX).
(Plant drawing from IFAS [52].)
The duckweed Lemna gibba was also found to be highly effective in removing ammonia from fish effluent, particularly when the water was circulated (Fig II-5). Ammonia levels in stagnant water rose during the first 20 h in both the plant-free pond and the one covered with duckweed. When the water was circulated, however, ammonia declined 90% within 48 h in the duckweed pond. In contrast, in the plant-free pond ammonia levels remained constant for the first 48 h after which there was a gradual decrease due to bacterial activity.
Figure II-5. Ammonia Levels in Fish Effluent in Ponds with or without Duckweed.
The ponds contained effluent from a large tank containing Tilipia fish. The water contained similar concentrations of nitrates and ammonium (0.08 mM of each). [In terms of mg/l, this would be 5.0 mg/l of NO3 and 1.4 mg/l of NH4.]
{Redrawn from Porath [73] and used with permission from Elsevier Science.}
E. Plants and Toxic Compounds in Aquariums
My own well water contains a small, probably harmless level of copper (0.05 ppm) but enough zinc (0.8 ppm) to sometimes cause problems in my aquariums. For example, when I did a large water change in one of my tanks with raw tapwater, the shrimp became agitated, scurrying here and there. I rescued two of the shrimp by immediately putting them into another tank, but the third shrimp, which I was unable to catch, died by the next morning. Also, some of the guppies became diseased within the next few days.
Plants are also affected. The Amazon Swordplants in my aquariums are slightly pale and contain very high levels of zinc. If I put Egeria densa into pure tapwater, plants quickly turn brown and decay. (I observed these same symptoms in an experiment where I grew the plant in nutrient media containing 1 ppm zinc.) When I grew Alternanthera in subsoil with a high manganese concentration, growth slowed and the leaves became crinkled and miss-shapened.
Some aquarium water conditioners contain the metal chelator EDTA, which is quite effective in counteracting metal toxicity. For example, I was able to neutralize zinc toxicity to Egeria densa completely by adding a very small molar excess of EDTA.9 One investigator [74] routinely added 5 mg/l of EDTA to prevent toxicity in experimental guppies.
Q. What factors would affect metal toxicity in aquariums?
A. Below are some of the measures that aquarium hobbyists often use that would be expected to reduce metal toxicity to their fish:
•Using R.O. (reverse osmosis) or deionized water for water changes
•Using aquarium water conditioners that can neutralize heavy metals
•Using peat filtration– peat binds metal ions in exchange for H+
•Using ‘Black Water Extract’– its humic acids would bind metals
•Increasing water hardness– calcium protects organisms from metals
•Fostering good plant growth and routinely pruning excess plant growth.
•Allowing DOC to accumulate
Cleaning measures that could increase metal toxicity are:
•Water changes– removes protective DOC, and if the tapwater is contaminated, each water change is, in essence, a fresh dose of metals
•Protein skimming– removes DOC
•Charcoal filtration– removes DOC
Although water conditioners containing EDTA provide short-term protection, plants provide long-term protection. I calculated that my plants take up about 13% of the zinc from the aquarium water each month.10 Although this removal seems small and hardly adequate, the plants are specifically taking up only the toxic form of zinc (Zn2+).
Finally, it is not just live plants that take up metals from the water and protect fish. Dead plant matter decomposes and eventually becomes humic substances, which bind and detoxify metals. Humic substances often give color to the water, but even if the water is colorless, humic substances may still be present.11
Aquarium plants– whether living or dead– protect fish from metal toxicity.
In aquariums both fish and bacteria continuously release ammonium as they metabolize food and organic matter. Fortunately for hobbyists, most aquatic plants (and algae) vastly prefer ammonium over nitrates as their nitrogen source. This means that plants continuously sift the water for ammonium and its toxic component ammonia. Thus, I’ve never had problems with ammonia in my planted aquariums.
Comment from Fish Breeder. I thought you might like to hear about my experience using plants in my breeding tanks. For 7 years I have been breeding and selling Angelfish wholesale to the aquarium stores in the local area. I sell about 2,400 per month, so I always have at least 100 tanks stocked with 100 to 500 fry of different ages.
For many years I’ve used homemade canister filters and do 50% water changes twice a week. If I don’t change the water, the fish quickly (within a week) begin to show what I call ‘ammonia burn’. That is, their long pectoral fins look ragged and chewed off. Sometimes the gill covers are missing, or the fish have ‘gill burn’.
A couple of years ago, by chance, I started adding Hornwort to some of the tanks. I’ve found that the fish in the Hornwort tanks need less care and water changes than in tanks without Hornwort. That is, the fish have less tendency to get ‘ammonia burn’.
Because I’m happy with the results of keeping plants in the tanks, I’ve installed additional lighting in my fish room and have started adding trays of planted Vallisneria to other tanks.
Hobbyists can protect fish from toxins by hard work, e.g., frequent water changes, gravel vacuuming, and enhanced filtration. However, given a chance, plants can purify the water naturally and effortlessly for the aquarium hobbyist. In my opinion, the ability of plants to purify aquarium water and protect fish has been woefully underestimated.
Hornwort or coontail (Ceratophyllum demersum). C. demersum is a rootless submerged plant. One successful fish breeder reported that the young fish showed less problems with gill and fin deformities when the tanks contained Hornwort.
{Drawing from IFAS [52].}
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1Heavy metals, except for aluminum, are classified as ‘Borderline’ and ‘Class B’ metals. In contrast, calcium and magnesium are ‘Class A’ metals and are generally not toxic [1,2].
2Fish get the majority of their calcium by absorbing it from the water through their skin and gills, not from digesting fishfood in the gut [28,29]. Both carp and trout readily extract calcium from water containing 5-20 ppm Ca [30].
3For an explanation of µeq (microequivalent), see ‘mg/l v. molarity v. equivalents’ on page 187.
4Not all metals bound to organic matter are less toxic. If the organic matter is hydrophobic (i.e., lipid soluble), it may act like an ‘ionophore’ in that it will actually carry the metal through the lipid bilayer into the cell. For example, mercury binding to methyl groups greatly increases its toxicity [2].
5Humic substances are random, nonspecific compounds resulting from the bacterial decomposition of plant matter (see page 61).
6Calculations: Log K of zinc (17.8) minus Log K of calcium (11.9) = 5.9. Antilog of 5.9 (e.g. 105.9) is 790,000.
7At pH 8.4, about 15% of this ammonium (NH4+) would be in the form of ammonia (NH3) [56].
8Hobbyist test kits measure nitrite, as has been done in this instance. However, scientists often quantify NO2- as nitrite-nitrogen (i.e., NO2-N), especially when they are comparing nitrite with other forms of nitrogen. [Later, I will show studies with this alternate quantification.]
NO2- is 30% N. Therefore, 100 mg/l of NO2- is equivalent to 30 mg/l of NO2-N. See ‘Abbreviations and Conversions’ on page 187.
9I neutralized the 2.0 ppm zinc (3.0 X 10-5 M) with 2.5 X 10-5 M EDTA. One mole of EDTA neutralizes one mole of metal. See page 187 for an explanation of molarity.
10I remove about 20 g (0.020 kg) of plants (dry wt) from my 50 gal (~ 200 l.) aquarium each month. The zinc concentration in these plants is 1,000 mg/kg, so the zinc removed from the tank each month is 0.020 kg X 1,000 mg/kg or about 20 mg. If the zinc concentration in 200 l of tapwater is 0.8 mg/l, then the tank begins with a total of 160 mg of zinc, because 200 l X 0.8 mg/l = 160 mg zinc. Thus, plant pruning removes 20 mg (~13%) of the starting 160 mg total zinc in the water.
11Humic substances strongly absorb UV light [75]. I checked the light absorption of water samples from several of my tanks. All samples absorbed little visible light but lots of UV light. For example, the colorless water from one of my tanks showed no absorption above a 400 nm wavelength, but at 225 nm the O.D. (optical density) was 1.9; at 200 nm, the O.D. was a robust 3.7. (I used quartz cuvettes with a 1.0 cm pathlength for this spectrophotometric analysis.)