"Goldfish (Carassius auratus) are small ornamental freshwater fish that are commonly kept as pets. Goldfish were one of the earliest breeds of fish to be domesticated and are still one of the most commonly kept fish in aquariums and outdoor water gardens.
Goldfish were originally domesticated from the Prussian carp (Carassius gibelio), a dark greyish brown carp native to Asia. It was first bred for color in China over 1,000 years ago. Due to selective breeding, goldfish have been developed into many distinct breeds and are now found in various colors, color patterns, forms and sizes far different from those of the original domesticated carp."
"Goldfish live natively in ponds, and other still or slow moving bodies of water in depths up to 20 m (65 ft). Their native climate is subtropical to tropical and they live in freshwater with a pH of 6.0–8.0,preferably a pH of 7.5, a water hardness of 5.0–19.0 dGH, and a temperature range of 40 to 90 °F (4 to 32 °C) although they will not survive long at the higher temperatures. They are considered ill-suited even to live in a heated tropical fish tank, as they are used to the greater amount of oxygen in unheated tanks, and some believe that the heat burns them[who?]. However, for centuries goldfish have been observed living in outdoor ponds in which the temperature often spikes above 86 °F (30 °C). When found in nature, the goldfish are actually an olive green color.
Fancy goldfish are unlikely to survive for long in the wild as they are handicapped by their bright fin colors; however it is not beyond the bounds of possibility that such a fish, especially the more hardy varieties such as the Shubunkin, could survive long enough to breed with its wild cousins. Common and comet goldfish can survive, and even thrive, in any climate in which a pond for them can be created. Introduction of wild goldfish can sometimes cause problems for native species. Within three breeding generations, the vast majority of the goldfish spawn will have reverted to their natural olive color. Since they are carp, goldfish are also capable of breeding with certain other species of carp and creating hybrid species.
While it is true that goldfish can survive in a fairly wide temperature range, the optimal range for indoor fish is 68 to 75 °F (20 to 24 °C). Pet goldfish, as with many other fish, will usually eat more food than it needs if given, which can lead to fatal intestinal blockage. They are omnivorous and do best with a wide variety of fresh vegetables and fruit to supplement a flake or pellet diet staple.
Sudden changes in water temperature can be fatal to any fish, including the goldfish. When transferring a store-bought goldfish to a pond or a tank, the temperature in the storage container should be equalized by leaving it in the destination container for at least 30 minutes before releasing the goldfish. In addition, some temperature changes might simply be too great for even the hardy goldfish to adjust to. For example, buying a goldfish in a store, where the water might be 70 °F (approximately 21 °C), and hoping to release it into your garden pond at 40 °F (4 °C) will probably result in the death of the goldfish, even if you use the slow immersion method just described. A goldfish will need a lot more time, perhaps days or weeks, to adjust to such a different temperature.
In the wild, the diet consists of crustaceans, insects, and various plants."
Source and further information:
http://en.wikipedia.org/wiki/Goldfish
