Koi (shortened from Japanese nishikigoi) are ornamental domesticated varieties of the common carp Cyprinus carpio.
While a Chinese book of the Western Jin Dynasty (4th century) mentions carp with various colors, koi breeding is generally held to have begun around the 17th century in the Niigata prefecture of Japan. Farmers working the rice fields would notice that some carp would be more brightly colored than others, capture them, and raise them (when normally the brighter colors would doom the fish to be more likely eaten by birds and other predators). By the 19th century, a number of color patterns had been established, most notably the red-and-white kohaku, but the outside world did not become aware of the degree of development until 1914, when the Niigata koi were exhibited in Tokyo, and some of them presented to Crown Prince Hirohito. At that point, interest in koi exploded throughout Japan and later worldwide. Koi are now commonly sold in most pet stores, with higher-quality fish available from specialist dealers.
Koi varieties are distinguished by coloration, patterning, and scalation. Butterfly koi, developed in the 1980s and notable for their long and flowing fins, are actually hybrids with Asian carp, and not considered true koi.
Koi coloration includes from one to three colors, the colors including white, yellow, orange, red, black, and blue (a grayish shade due to black underneath the skin), with either a flat or metallic appearance. Patterning is infinitely variable, but desirable patterns include a round patch on the forehead, and a stepping-stone pattern down the back. The scales may be present or missing, large or small, or crinkled, giving a "diamond" appearance.
While possible variations are infinite, breeders have identified and named a number of specific types. The breeding process is complicated, since most of these varieties do not breed true, and much of the knowledge is still a closely guarded secret. Individual fish are also selected for appearance while being raised; the net result is that an aesthetically attractive fish of a rare variety may fetch a price of hundreds or even thousands of dollars.
Major koi varieties
Asagi - light blue on top, red/orange on bottom, blue scales bordered in white
Shusui - similar to asagi, but with large scales in a dorsal row
Bekko - primary color red/orange/yellow/white, with black patches
Hikarimoyo mono - two colors; one flat, one metallic
Hikari utsuri mono - two metallic colors
Kawari mono - miscellaneous
Goshiki - mostly black, with red, white, brown, and blue accents
Kinginrin - bright metallic sheen, silver highlights
Kohaku - red accents on white body
Koromo - red and white overlaid with blue or silver
Ogon - uniform yellow or white
platinum ogon - pure white
Showa sanke - black with red and white markings
Taisho sanke - primarily white, with red and black markings
Utsuri mono - uniformly black, with red, white, and yellow markings
Tancho - primarily white, with a red patch on the forehead
Tancho kohaku - pure white, round red head patch
The common carp is a hardy fish, and koi retain that durability. They can be kept in anything from small containers to large outdoor ponds, although they quickly grow to 30 cm (1 ft) and can get much bigger, so the traditional indoor aquarium is less desirable than a round plastic tub. Koi are basically cold water fish, so it's advisable to have a half-meter or more of depth in areas of the world that become warm during the summer.
Koi's bright colors put them at a severe disadvantage against predators; a kohaku looks like a visual dinner bell against the dark green of a pond. Herons, kingfishers, raccoons, cats, foxes, and badgers are all capable of emptying a pond of its fish. A well-designed outdoor pond will have areas too deep for herons to stand in, overhangs high enough above the water that mammals can't reach in, and shade trees overhead to block the view of aerial passersby. It may prove necessary to string nets or wires above the surface.
The pond should include a pump and filtration system to keep the water clear.
Koi are bottom-feeders, so koi food is not only nutritionally balanced, but designed to float so as to encourage them to come to the surface. Koi will recognize the person feeding them and gather around at dinnertime. They can even be trained to take the food from one's hand. In the winter their digestive system slows nearly to a halt, and they eat very little, perhaps no more than nibbles of algae from the bottom, and their appetite won't come back until the water warms up in the spring.