One of the more popular breeds of fish for the backyard pond is koi. They are a member of the carp family and are related to the European common carp (Cyprinus carpio). Interest in koi may range from just having a few colorful fish swimming around your pond to the more serious enthusiast who breeds and shows the fish in competitions.

Koi have been bred in Japan since around 300 A.D. and have become very popular throughout the world. They have been divided into many different categories based on their color pattern and size. Each group has a distinct Japanese name which is used by growers to identify individuals for showing.

Although special "show koi" may cost hundreds and even thousands of dollars, plenty of these fish are also available at reasonable prices for the backyard pond. Perhaps the key consideration that separates koi from most of the other pond fish is their size. Over time, they may reach a length of 2 to 3 feet and become substantial presences. This means that you need a suitably sized pond to hold them. 

A pond of at least 6 feet in diameter is generally considered a minimum for koi. This allows enough room for even the larger ones to maneuver easily. While a depth of 18 inches may be adequate for goldfish, koi should have at least 2 feet of water. Three feet or more is better in most cases to minimize rapid temperature changes in the pond.

If you just want a few fish in the pond, you can treat the koi the same as any other fish. However, if your goal is to grow championship caliber koi, you may need to have a more elaborate filtration system and pay much more attention to their nutritional needs. Special feed formulations are available which are supposed to encourage brighter coloration and rapid increase in size for these fish.