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.
Comet (goldfish)
The Comet is more active than most other goldfish breeds. It is not unusual to see a Comet dashing back and forth in its tank, racing around in a playful manner. Due to the Comet's hardy and active nature, and the relative ease in caring for them, they are the breed best suited to ponds and outdoor pools. They are often kept with koi in outdoor ponds. Comets have a natural life span of 5 to 14 years and may live even longer in optimum conditions.Goldfish are commonly bred on fish farms in many parts of the world. In most instances, the fish produced are offered for sale to aquarists. However, in North America, there is a demand for goldfish used as bait or "feeder fish" to other fish by anglers. Due to the relatively inexpensive prices of comet goldfish, they may also be used as prizes in carnivals or other places of entertainment. The use of goldfish as bait, feeder fish and carnival prizes is controversial, and animal rights activists have attempted to make the practice illegal, albeit unsuccessfully.