Fish are not necessary for the balance of the pond, but their presence will greatly increase the speed that it is established. Further, fish eat many of the undesirable visitors to your pond, keep submerged plants pruned, recycle nutrients in the system, and add immeasurably to the beauty of the pond. Fish should be stocked at a rate no more than 1 inch of fish per 3 to 5 gallons of water in the pond. Fish attain a greater size, do less damage to submerged plants, and remain healthier if stocked at rates below the capacity of the pond. Thus a small circular pond 5 feet in diameter and 18 inches deep holding around 160 gallons of water will accommodate from four to five 10 inch fish. When first introducing fish to the pond, put the fish still inside their sealed bag into the pond allowing 15 minutes for the temperature to equalize between the bag and the pond before releasing the fish. If it is a sunny day cover the bag to prevent overheating.
Japanese koi are carp. These fish are generally expensive, but range in mature size from 2 to 3 feet, come in many colors and live for a very long time. Koi are best suited to ponds larger that 6 feet minimum diameter and at least 18 inches deep.
Are smaller and by far the most popular fish for pond use. They are less expensive, come in many colors, and range from 10 to 12 inches at maturity. Goldfish will breed in your pond thus increasing in numbers over a period of years. This may present a problem for small ponds.
Frogs are good for the pond; they supply tadpoles which are efficient scavengers and food for fish and dragonfly larvae. Also, the adults in conjunction with the fish control the mosquitoes and other insect problems.
There is some controversy over including snails in the pond. The Japanese black snails may not help keep the water cleaner, though they do slowly eat away at the algae. They are, however, fun to watch for and they will not get out of the pool and damage any other plants in your garden.
In most instances, if you have stocked your pond properly and maintain the health of your pond inhabitants, there may be the occasional unwanted “Diving Water Beetle” or “Water Boatman,” but these and others pests can be kept in check by the predation from larger fish and frogs.