The Most successful garden ponds are between 18 and 24 inches deep. Very small ponds can be as shallow as 12 inches; very large ponds can be up to 36 inches deep. The 18 to 24 inch depth is considered optimum for growing water lilies and other aquatic plants; this is sufficient, too, for raising most types of fish and other aquatic life. Also, except in the coldest climates, a depth of 24 inches is sufficient to keep the pond from freezing all the way to the bottom, which in turn would kill fish or damage the pond shell. If installing a Japanese koi pond, you may need to provide an area at least 3 feet deepwhere fish can excape heat in the summer and frozen water in the winter. Small, shallow ponds heat up and cool off more quickly than large ponds. This can stress out the fish, and possibily kill them.