Fish, such as Koi, Comets, and Goldfish can be successfully kept, and even bred, in water that is only two feet deep as far north as Zone 4 (MN). The fish keep the pond clean by eating excess algae that may try to form on the rocks of the pond as well as organic material that finds its way to the bottom. You never need to feed the fish in a system like this, although you probably won't be able to resist tossing them a handful of pellets every now and then. The ammonia in the fishes' waste is converted by the bacteria into ammonium nitrate, which is fertilizer for the plants! They look beautiful and I wouldn't have a pond without them, but the fish also play an equally important role in the balanced pond ecosystem.
