The typical water garden of 2 to 2 foot depth is best stocked with goldfish. Koi, including butterfly koi, require more depth for proper vertical swimming exercise. Do not purchase fish if other fish in the tank are sick. Sick fish may have dull eyes, inflamed skin, frayed fins, or isolate themselves from the rest of the fish in the tank. Any new fish to be added to the pond should be kept in a separate quarantine tank for 7 to 10 days and observed for any health problems.
Snails are the intermediary host for anchor worm.
If you have snails in your pond, check fish in the early spring for this parasite. Feed fish only what they will eat in 5 to 10 minutes. If you feed floating fish food, you can be better assured that food is not ending up on the pond bottom where it will decompose and possibly foul the water. Fish need high protein food in the summer. In the spring and autumn, feed them wheat-germ based foods that are easier for their slowed metabolisms to digest.