Fish
As soon as you add the water, natural wildlife will find the pond. When I built my first pond, I had a frog in it the day after it filled. He must have been waiting for me to finish construction. This type of wildlife is all you really need. Fish do not have to be added, but they do provide a lot of extra enjoyment, and for some people, fish are the main reason for having a pond.
Fish can be detrimental to the pond. They like to eat the plants and the wildlife that comes to the pond. They will eat tadpoles and frog eggs, reducing the potential native populations. However, they also eat mosquito larvae, which is beneficial. Fish are a major contributor to nutrient overload that causes algae growth. Increasing the number of plants will overcome this problem.
Fish need three main things to stay healthy: oxygen, food, and low levels of ammonium. The best way to provide these is to maintain a small amount of fish. Fewer fish use less oxygen and produce less waste. The ammonium in the water comes from fish waste; less ammonium keeps fish healthier.
Food is not really a problem in a natural pond. Unless you are raising game fish, you don’t need to feed the fish at all. Let them find their own food. My goldfish never get fed. They grow quickly and find lots of food. If insect populations are low, they will eat more algae.
It is not the number of fish but their weight that counts, although most people talk in terms of fish length because it is easier to measure. A ten-inch fish is about equal to ten one-inch fish. Fish grow, so you need to consider their adult size. Some types will breed in the pond, and you should allow extra room for the babies.
How many fish can you add? As a general rule, keep your fish load under one inch of adult fish, not including the tail, for every one square foot of surface area. Less is always better. Adding more fish will increase the strain on plants to keep the water clean. In the natural pond, it is essential to balance the amount of fish with the number of plants.
If you have just filled the pond, don’t rush to add the fish. Give the water a week to settle and allow any chlorine to dissipate into the air. This also gives insects time to find the pond so the fish have something to eat. When you bring the fish home, float the bag in the pond for about 30 minutes to let the temperature inside the bag reach that of the pond. Then release the fish.
Some people recommend adding salt or other tonics to a new pond, especially for koi. Salt can help fish fight disease, and it helps fish swim easier in the higher-density water. Unfortunately, salt is not good for plants. Salt and tonics are not needed in a natural pond. Remove any sick fish and treat them in a separate container.
Many articles talk about adding bacteria to “charge” the pond, claiming that they are needed to maintain low ammonium levels. It is true that bacteria do this, but it is not true that you need to add them. They start colonizing the pond even before you finish building it. The rocks, plants, and even your dirty hands have added billions of bacteria. Don’t add more.
The following sections describe the more common fish in man-made ponds, and you do not have to select just one type. Except for the mosquito fish, which will become food for the larger fish, you can include several types. Overwintering fish will be discussed in more detail in the chapter on pond maintenance.
Goldfish, an excellent choice for most backyard ponds, are highly recommended if you have never had fish before. They are inexpensive, will find their own food, and can overwinter outside in most regions. They are easy to maintain, will breed in small ponds, and provide a lot of enjoyment.
You can buy fancy goldfish for your pond, but most people buy feeder fish, one- to two-inch fish sold, by the dozen at a low price, at pet stores as food for larger fish. The problem with feeder fish is they are not all very healthy, and in my experience, about 30 percent will die in a day or two. The rest will grow and prosper in the pond, so it is still a good deal, and you save 70 percent from being eaten.
Goldfish need to be about four inches long before they will breed. They might breed in the first year, more likely in the second spring. After a while, you may have too many fish, but a visit from the local heron or raccoon will solve the problem. If they don’t eat enough fish, you might need to remove some so that they don’t produce too much fish waste.
In most regions, goldfish will overwinter, provided the pond is deep enough not to freeze solid. Most people just leave them in the pond to fend for themselves over winter.
There are also many fancy goldfish for you to try. Comets have straight, darting bodies of red-gold. Fantails with their flowing fancy tails move gracefully through the water. The black Chinese Moor has telescopic eyes.
Koi are very popular and make good fish for medium- and large-sized ponds. They are available in a magnificent variety of colors and patterns, including metallic and multicolored. They can be trained to eat out of your hand and to come when you call them.
These fish grow much larger than goldfish, and many people end up with too much fish for the size of their pond. Koi grow to two feet long, and as adults, they each require 100 square feet of surface area. Each fish should have at least 300 gallons of water. They also require cleaner water and higher oxygen levels than goldfish, which makes them a bit harder to keep. The key is to keep fish levels low.
If you are considering koi, you probably need to decide if your interest in a natural pond is because you want the pond or because you want to keep koi. The two are not perfect partners although they can co-exist. The problem is that koi like to eat plants, and they disturb roots just for the fun of it. They also produce a lot of fish waste, in part because koi owners like to baby their fish and feed them a lot.
A pond with koi that continually disturb the plants will end up with algae problems. One solution is to physically separate the plants from the koi by making the inner edge of the planting shelves higher so that the fish can’t enter the area where plants grow. It will still look like one pond, but underwater it will have several distinct sections.
In colder regions, most koi are taken inside for the winter. If they are left outside, the pond should be at least four feet deep; many koi owners prefer five feet.
Another issue with koi in natural ponds is that herons like to come for a meal. Since koi can be expensive, most owners are not too happy when this happens. Some will even keep their ponds covered with netting all of the time. This works but does not look very natural.
You can certainly keep koi in your natural pond, but they are probably not the best choice.
Mosquito fish, small guppy-like fish in the genus Gambusia, are native to North and Central America. They grow up to two inches long and consume large quantities of insect larvae as well as algae. They are mostly silver or light brown and breed easily in small ponds. They are not as interesting to look at as goldfish or koi, but they do a great job keeping the pond free of mosquitoes.
There are several different species of mosquito fish. If you live in colder climates, get some local varieties that are much more tolerant of cold than southern species. Northern mosquito fish can overwinter in the north; southern varieties will not. They can be difficult to catch, so it is best to leave them in the pond over winter.
Unlike koi and goldfish, males and females are easy to differentiate. The males are smaller, thinner, and have a pointed anal fin, like that of a guppy. Females have a dark pregnancy patch on the lower portion of the body.
The golden orfe (Leuciscus idus) is native to Europe, where it inhabits lakes and slow-moving streams. They are very active, orange or golden, and spend much of their time at the surface of the water. Unless kept in a school of at least six, they tend to be shy.
Because adults are almost two feet long, they need to have a larger pond. A school of six fish make a spectacular display. They are not easy to breed, which can be a good thing since too many baby fish can be a problem. They seem to only breed when they get older.
Like koi, orfes require higher levels of oxygen. This is not a problem in a large pond in summer but may be in winter. They can overwinter in ponds that do not freeze solid, but such ponds can have oxygen levels that are too low. For this reason, it might be best to bring the fish inside in zones 6 and lower. To better understand hardiness zones, consult the link provided in the References section.
Game fish are selected based on the temperature of the water, size of pond, and availability of breeding areas. Each type has a preferred habitat for breeding, and if this is important, make sure the pond is designed for your chosen fish.
Small ponds will only accommodate one type of fish. Larger ponds, over an acre in size, can accommodate two or even three. These also provide more variety in habitats and temperature ranges.
Fish have a preferred range of temperature, and if it gets too high, they will get sick and die. Cold water fish need water that never goes over 70°F. Cool water fish can have temperatures go above 70°F, but they should rarely reach 80°F. Warm water fish live comfortably in summer temperatures up to 90°F.
Stock the pond with fish from a breeder to reduce the chance of introducing a disease into your pond. In some areas, it is illegal to stock a pond with captured fish. Avoid adding the following fish: crappie, bullhead, yellow perch, pumpkinseed, carp, sucker, flathead catfish, blue catfish, and green sunfish.
The largemouth or black bass is a primary sport fish. Growth rates vary depending on pond conditions, density, and genetics. They can reach ten inches by their third year. Their diet consists of insects when they are small and changes to fish as adults. These warm water fish spawn in spring, at 60°F, in the shallow margins of the pond.
The bluegill is considered a secondary sport fish and may be feeders for other types of fish. They can grow up to six inches in their first year in optimal conditions, maturing at eight inches in their second or third year. This warm water fish spawns in shallow water at about 75°F and prefers to eat insects.
The redear is a secondary sport fish with growth rates and spawning habits similar to the bluegill except that they produce fewer young. These cool water fish eat insects and snails at the bottom of the pond and are considered a biological control for a snail parasite that causes swimmers itch in humans.
The channel catfish, prized for their eating quality, thrives in small ponds. They spawn once temperatures reach 72°F and continue spawning for the summer. These warm water fish get along with other types and are routinely used as a secondary fish. They eat just about anything, including crayfish, insects, and small fish.
This minnow is used as a forage fish for the largemouth bass and reaches a length of four inches. They have warm water spawning habits and may spawn in a pond.
This cold water fish is an excellent choice, provided the temperature never goes above 70°F. They grow most rapidly between 50° and 65°F. Usually stocked at six-inch fingerlings, they can weigh eight ounces at the end of the first year. Like channel catfish, they readily take pelletized food and can be maintained in small ponds. Rainbows rarely reproduce in recreational ponds and must be restocked.