Carp, along with many of their cyprinid relatives, are popular ornamental aquarium and pond fish. The two most notable ornamental carps are goldfish and koi. Goldfish and koi have advantages over most other ornamental fishes, in that they are tolerant of cold (they can survive in water temperatures as low as 4 degrees Celsius), can survive at low oxygen levels, and can tolerate low water quality.
Goldfish (Carassius auratus) were originally domesticated from the Prussian carp (Carassius gibelio), a dark greyish brown carp native to Asia. They were first bred for color in China over a thousand years ago. Due to selective breeding, goldfish have been developed into many distinct breeds and are found in various colors, color patterns, forms and sizes far different from those of the original carp. Goldfish were kept as ornamental fish in China for hundreds of years before being introduced to Japan in the 15th century, and to Europe in the late 17th century.
Koi are a domesticated variety of common carp (Cyprinus carpio) that have been selectively culled for color. The common carp was introduced from China to Japan, where selective breeding of the common carp in the 1820s in the Niigata region resulted in koi. In Japanese culture, koi are treated with affection, and seen as good luck. They are popular in other parts of the world as outdoor pond fish.