Koi carp grow to an average size of 24 to 36 inches long. It is possible for some to reach lengths greater than 4 feet but only when pond conditions replicate their natural environment and they have plenty of space.

Native to China and Japan, koi carp should be kept in ponds a minimum of 4 feet deep, and there must be at least 10 gallons of water for every fish. Since the species produces a lot of waste, a filtration system is a must, and at least 10 percent of the water should be changed out each month to control the amount of ammonia, nitrates and other toxins in the water.