Koi are the most common pond fish. They are originally bottom feeders bred from an Asiatic carp by Japanese rice farmers. They will for the most part be limited in size by the size of the pond they are in, though there will be some sports who do outgrow their environment. Under ideal conditions they can grow to about three and a half feet long. Most of their growth will be in their first three years. If they are kept exclusively in a small pond when young, they will never reach their potential full size. They will start spawning when about two to three years old. They usually spawn in spring, typically in March, April or May. Koi and goldfish can interbreed, though they do so infrequently.
Goldfish were bred from European carp and do very well in an ornamental pond. They can attain a length of up to two feet in a large enough pond. They are prolific breeders and will spawn their first summer if large enough, and spawn in spring, summer and fall. Though not as colorful as koi, they have much more variety in body shapes. Not all of the very fancy types do well outside, particularly in the winter. Comets, Sarasa comets, Veiltails, Fantails, Calicoes and Shubunkins are all goldfish varieties that make excellent pond inhabitants.
Golden Orfes –
Golden Orfes are a top feeding schooling fish. They are completely unrelated to goldfish and koi. They are native to Europe and exist there in both golden and silver versions. The silver version is seldom available in the U. S. They should be kept in schools of at least 4 to 6 fish and will typically get a foot to a foot and a half in length. The are incredibly hyper and are always flitting about the pond. They particularly enjoy leaping up into waterfalls and leaping after gnats and other insects. Due to their constant activity and jumping, they should be kept in a pond of at least 1000 gallons; with surface area being more important than volume (a very shallow 800 gallon pond is better than a deep 1200 gallon one.)
Mosquito Fish –
Mosquito fish (Gambusia sp.) grow to about two inches for females and one inch for males. They are livebearers and breed prolifically. They are excellent for container gardens or small bodies of water where bigger fish cannot get, as koi and goldfish will also eat mosquito larva. They are fun in a large pond anyway, as they are entertaining to watch as they eat. They will easily winter over in an outside pond in this area.
Channel Catfish –
Channel catfish are typically available in either blue (natural) or gold (albino) versions. Either will do fine in an ornamental pond, provided that it is understood that they are not necessary, as, contrary to common belief they do little or nothing to clean your pond. Koi are better scavengers than catfish. Large catfish can be ornamental, but will usually eat anything that will fit in their mouths, including small fish. Small catfish become large ones in a year or two at most. They do an excellent job of controlling goldfish overpopulation.
European Dace are a large surface fish, very similar to a silver Orfe. These are seldom available in the U. S. American Dace are a much smaller minnow type fish. Dace, killifish and other small minnow type fish will frequently show up in ponds from eggs that come in on plants, notably Water Hyacinths, Water Lettuce, Anacharis and Cabomba. They are harmless and add to your biodiversity, but are seldom available commercially.
Many varieties of snails are available for your pond. Small snails, both pond snails (long pointy shells) and ramshorn snails (flat round shells) will show up in your pond without you doing anything. They arrive as eggs on plants and as eggs or babies on birds’ feet. They will do very little to clean your pond and will eat your plants, but adult koi and goldfish will eat these snails. You will notice them most in skimmers and filters where your fish cannot get at them. A mid sized snail for ponds is the Columbian Ramshorn snail. They typically don’t bother plants and add a little color with their striped shell. Larger snails for a pond are Japanese Trapdoor snails, Mystery Snails and Apple snails. These three varieties all look very similar. Apple snails are the largest and also the most voracious plant eaters. Mystery snails do some plant eating and some algae while Japanese Trapdoor snails are almost exclusively algae eaters. Japanese Trapdoor snails also breed slowly enough not to overpopulate a pond.
Freshwater Clams –
Freshwater clams were common years ago to help keep ponds clean, as they filter a tremendous volume of water through their bodies. However, to keep them you need a bed of relatively clean sand on the bottom of the pond. With the advent of modern reliable pumps, clams are now seldom kept in the backyard pond.
Crayfish & Shrimp –
Crayfish and freshwater shrimp are frequently introduced to ponds and will help clean your pond, but are also eaten and / or picked on by the larger fish and so don’t really do well in the typical backyard pond.
Frogs & Tadpoles –
Tadpoles are a frequent addition to backyard ponds. Those usually available for sale are bullfrog tadpoles, which stay tadpoles for about two years. After becoming frogs, they will grow very rapidly to a large size. On occasion frogs may eat smaller fish like mosquito fish and young goldfish or koi. Frogs of all types may show up in your pond on their own. Toad tadpoles, which are small (about ½” long) and black frequently show up in ornamental ponds, but only remain tadpoles for about three weeks before becoming toads. Tree frog tadpoles also frequently show up in ponds, taking several months to grow into frogs.
Turtles, such as red-eared sliders or painted turtles, are a fun addition to your pond. They are entertaining to watch, though they will munch on your plants and eat very small fish, making them a better addition to a large pond than a small one. They are also more likely to stick around in a large pond. They need to have someplace to get out of the water to sun themselves to prevent shell rot.
Many tropicals can be placed in your pond during the warm summer months. The most commonly placed in a backyard pond are plecostomus catfish, used as algae eaters. They may grow up to two feet long in an outdoor pond. They must go back inside by late September. Swordtails and mollies are also frequently put outside for the summer.
Native fish can be put in ornamental ponds but it not usually recommended. Wild stocks are frequent carriers of disease, many wild fish are voracious predators and in many states relocating live wild fish is illegal. If native fish are to be introduced to the pond, it is recommended that they come from a hatchery.
Chinese Hi-Fin Sharks
Hi-fin sharks are a cold water hardy algae eater in the catfish family that will do well in an outdoor pond. In a large enough pond they can easily reach a foot and a half in length, but even at that size won’t bother other fish, even little ones. As they are not yet bred commercially, but are all wild caught in China and imported they tend to be somewhat pricey, but they are fun to watch. The intensity of their coloration and stripes seems to vary considerably with mood, temperature, time of day or other unknown factor.