Goldfish (Carassius auratus) are probably the most commonly kept pond and aquarium fish in the world and probably the first domesticated fish. In the west, goldfish are thought of as inexpensive children's' pets costing as little as 20 cents to a few dollars for a pet-grade fish. However it is not uncommon for collectors to pay thousands of dollars for a single well-breed, show-class fish. In Asia keeping goldfish is a very honorable and respected pastime and goldfish are often referred to as living art like bonsai.

Goldfish live, on the average, 5-10 years but have been recorded living as long as 49. They can grow up to 24 inches long, but rarely do so. Most "fancy" breeds won't grow longer than nine inches.

They come in a variety of colors including: orange; red; yellow; brown; blue; black; white; green and purple and can be solid, multi colored or calico. Scales are metallic, matte, or nacreous. Unlike koi, different breeds of goldfish have many variations in body and finage—so much so that different breeds may look like different species. Goldfish bodies vary from long and slender (wild-type) like a koi to almost spherical in the case of the ping-pong pearlscale. Goldfish tails may be single or divided into two separate fins, short or long. Other body variations include: lack of dorsal finage; nasal growths; wen (fleshy head growth); telescope eyes; bubble eyes; and pearlscales.

Because of their relatively small size, goldfish can be kept in shallow water so long as the water temperature is kept stable and cool—in fact, some breeds, such as tosakin, should only be kept in shallow water.

Goldfish, like koi, are social fish and swim together in schools. Although goldfish can be kept with koi, will school with koi, will eat the same food as koi and can even interbreed with koi, they are different species. Goldfish are subject to the same diseases and parasites as koi and can be treated for illnesses much as you would koi. The hibuna (common), comets, wakin, jikin, shubunkins and other long bodied breeds are strong swimmers and very hardy and can be kept with koi. Others short-bodied or "fancy" breeds should be kept with breeds with similar swimming ability and size. Tosakin and bubble eyes are not appropriate for most natural-style or planted ponds and should be kept in specialized ponds (shallow stock tanks) or aquariums to protect fragile body structures and will therefore need winter protection.

It is often thought that all fancy goldfish need to be brought in for the winter as they are not as tolerant of the cold as the wild-body-type breeds such as comets. Though not quite as hardy as a koi, most breeds of goldfish can easily over winter under ice providing there is adequate gas exchange. Goldfish tolerate temperatures below 40F and above 90F but do best at temperatures in the 60s and 70s. If kept indoors, goldfish should be kept without supplemental heaters.

Goldfish are top and mid-water feeders with forward facing mouths (in contrast to the bottom feeding koi's downward facing mouth) and are effective at controlling mosquitoes and other aquatic insects. Though they will graze algae and eat off the bottom, they will not dig through potted plants or eat the leaves as koi will often do.

There are dozens of, if not more, breeds of goldfish—only a fraction of which can be found in the West. Bristol Aquarists Society have put together a collection of descriptions and pictures of the most common breeds. Wikipedia is another good source of information about the different breeds and their histories. The most comprehensive websites about goldfish are naturally Asian as goldfish keeping is much more popular and respected in the East.