Koi
The major named varieties include:Goldfish (金魚) were developed in China more than a thousand years ago by selectively breeding Prussian carp for color mutations. By the Song dynasty (960–1279), yellow, orange, white, and red-and-white colorations had been developed. Goldfish ("Carassius auratus") and Prussian carp ("Carassius gibelio") are now considered different species. Goldfish were introduced to Japan in the 16th century and to Europe in the 17th century. Koi, though, were developed from common carp in Japan in the 1820s. Koi are domesticated common carp ("Cyprinus carpio") that are selected or culled for color; they are not a different species, but a subspecies, and will revert to the original coloration within a few generations if allowed to breed freely.
Common goldfish
The sun is important to introduce to the goldfish frequently; due to the fish living in well-lit conditions in the normal wild environment, it is necessary to keep a similar environment for the fish. The light will also help the goldfish with its sleeping cycle, giving the goldfish a healthy sleeping schedule to stay healthy. The goldfish should not be kept all day in the hot sun, as this could possibly kill the fish and make the environment hostile. A tank light is available as an alternative but not necessary.