The common carp is the forerunner of our present day Koi. Koi are not big goldfish. The goldfish is a distant cousin to Koi. The Japanese name for Koi is Nishikigoi. Nishikigoi were developed by the Japanese over 200 years ago. These gorgeous creatures usually live in lushly landscaped fish ponds outdoors. The ponds are constructed to provide adequate oxygenation and filtration of the water. These ponds make an excellent landscaping addition to a garden. The average Koi can grow to 24 - 36 inches! The size of the pond, the amount of aeration, and feeding methods will affect the growth of the fish. It is not uncommon for a small Koi to grow 2 - 4 inches a year in a backyard pond.