If you decide to add fish to your pond then consider that like water plants, fish should be introduced to the pond during the warmer months. In cooler areas, fish become semi-dormant during the colder months and are more prone to stress when moved. Don’t add the fish right after you finished to plant your pond. Allow at least two weeks after planting for the pond to settle down and the plant roots to establish before stocking your pond with fish.
Do not be tempted to add too many fish. Overstocking with fish may encourage algae which feed on excess waste. To make sure you add the right number of fish to your pond calculate a maximum of 50 cm of fish body length, considering this is the length when fully grown, per square meter or yard of water surface. If you are going to add only small fish then consider 5 cm of fish body length per 1000 square cm. For better results add supplementary filtration.
It is advisable to add the fish to your pond in two stages, introducing half the fish 8 to 10 weeks before the rest. This process will allow the bacteria that feed on the fish waste products to multiply to a sufficient level to deal with the waste. If you are introducing too many fish at ones, the pond water will become polluted and the fish starved of oxygen. To provide a temporary relief you may leave a fountain or waterfall on overnight to agitate the water.
Usually you will but fish in a large clear plastic bag containing a small amount of water and inflated with oxygen. Do not release the fish straight away into your pond. The water temperature from the bag may be much warmer than the water from your pond and fish are very sensitive to sudden changes in temperature. Let the bag with fish to float unopened on the water surface of your pond until the water temperature from the bag is the same as in the pond. In hot, sunny weather, shade the bag with a cloth. After the water from the bag has the same temperature as the pond water, a small amount of pond water may be gradually introduced into the bag before you release the fish into the pond. Do not lift the bag to examine the fish closely because this will cause them extreme stress.
Except from fish, there are other water creatures that you may introduce to your pond, many of them as scavengers that will help you keep the pond clean. Ramshorn snails are often sold specifically for this purpose. Ordinary pond snails may become a nuisance as they eat the water lily leaves. Another addition are mussels, and in particular the swan mussel, which are excellent scavengers. They will clean up where fish have been overfed. Mussels will require deep water that remains cool even in the hottest summer days.