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.
Pristella maxillaris
Older aquarium books often refer to this species by the junior synonym "Pristella riddlei". Aquarists tend to refer to this fish as the golden pristella tetra, though some call it the X-ray tetra, instead. Other common names include goldfinch tetra, a reference to the similarly coloured goldfinch, and pristella tetra, a modification of its Latin name. A common name that was once widespread but is now rarely used is signal tetra, a reference to the similarity between the yellow and black dorsal fin and the arm of a semaphore railway signal.