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
"Fin Rot" is developed from bacteria eating/ deteriorating the fins of the goldfish. The disease starts from “poor water quality, overcrowding, sudden temperature changes, fin nipping, or aggressive fish”; this will lead to torn and shredded fins that could possibly deteriorate the body tissue to prevent the regrowth of their fins. Fin rot is known to be treated early to prevent further damage to the body tissue to the fish so the disease does not spread. Treatments start with clean water, adding aquarium salt to clean the water; if the following does not work then commercial products should be used along with weekly water changes.