The colorful koi fish, called nishikigoi in Japanese, is a specially bred variety of carp. Carp can be found all over the world in cool water, and are pursued for sport and for food. In the 19th century, Japanese farmers began breeding decorative carp, selecting brightly colored specimens to ornament gardens in luxurious fish pools. Koi can be found all over the world in decorative gardens and in a dizzying variety of colors, shapes, and sizes. Although they resemble oversized goldfish, sometimes growing up to three feet (91 centimeters) in length, the two fish are related only distantly.

In Japan, the term “koi” refers to carp. The fish known in English as koi are actually nishikigoi, “brocaded carp,” a reference to their colorful patterns. In Japan, the fish are considered to be lucky symbols of friendship and affection. Koi were introduced to the world in 1914 during an exposition, and quickly became popular decorative pets, especially when the introduction of plastic bags and rapid shipping made shipments of the valuable fish viable. Some particularly distinctively patterned koi sell for thousands of dollars.

In Japan, koi are differentiated by pattern. The most popular is the Kohaku, a white skinned koi with a red upper pattern. Other varieties include the Tancho, a white koi with a red dot on its head, Showa, a black koi with white and red spots, and Ogon, a solid colored koi which can be flat or metallic. Numerous other named types of koi can be found at dealers, and more are constantly being introduced through careful selective cross-breeding and breeding with wild carp.

Koi are cool water fish, and prefer deep ponds so that they can sink to the bottom in hot weather. They can be lured to the surface of a pond with food, and can also be taught to recognize people and taken food from their hands at feeding times. People who keep koi feed them with specially balanced koi food that floats, encouraging the fish to surface so that they can be checked for potential signs of illness.

Most Japanese ornamental gardens feature a koi pond which is surrounded by lush foliage and benches to sit and contemplate. Koi pools can also be found in a variety of other locations, and the hardy fish do well in a wide range of environments. Koi breeders recommend that koi pools be shaded, so that they do not grow too warm and so that the brightly colored fish cannot be seen from overhead by predators. In addition, a koi pond that is not protected from predator species should be built with overhangs for the koi to hide under.