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
Goldfish diets can be a range of pellets, flakes, boiled soft vegetables, and blood worms. Overfeeding can result in throwing off their biological balance, increasing the risks of diseases. The common goldfish should be fed “a small pinch of food two or three times a day…” to prevent extraneous amounts of food dirtying the water; this will also depend on the size of the fish of knowing to feed the fish more or less to keep healthy. Filtration will help prevent the bowl from getting dirty more quickly. A filtration system also helps produce good bacteria for the fish to make the water at safe levels. There are different types of filtration systems, ranging from the store bought external or internal filters or the use of aquatic plants. The reason a filtration system becomes helpful is due to keeping the nitrate levels low; ammonia is introduced in the water when the fish excretes which develops into nitrite into nitrate, leading into possible nitrate poisoning.