Don't feel bad if you can't tell the difference between koi and goldfish: The two species resemble each other closely. They have several important differences, though, and these differences translate to some major differences in care. Learn to reliably tell these two fish apart.
carp on Steroids image by Christine from Fotolia.com
Both fish originate in east Asia and have a long history of selective breeding. This breeding has produced fish that barely resemble their drab carp ancestors. However, the goldfish (Carassius auratus auratus) originated in China and has a history going back centuries, if not millennia. It is among the first domesticated pet fish. In contrast, the koi (Cyprinus carpio) comes from Japan and was domesticated as late as the 19th century. Despite each fish coming from a difference genus, the two are rather closely related.
goldfish image by martin schmid from Fotolia.com
In adult specimens, the quickest way to tell apart the two species is by their size. Goldfish can grow to a foot in length, though is very rare to see one this size. Many are kept under less-than-optimum conditions, which often stunts their grown. Additionally many strains (breeds, types) simply don't grow that big. Koi are much larger -- they can reach a length of 3 feet at maturity.
Kinkakuji image by Pixler from Fotolia.com
You can also tell goldfish from koi by their habitat. This method is not surefire, but it's a good starting point. Generally, people keep goldfish in aquariums or bowls, though aquariums suit them better. Koi thrive in ponds at least a meter deep. However, some people do keep goldfish in ponds. At the same time, a huge aquarium could hold koi, particular juveniles. Pet shops often sell very young koi from fish tanks.
Goldfish image by Lucy Cherniak from Fotolia.com
Selective breeding has produced a wide variety of goldfish and koi. However, koi color patterns have changed, while goldfish varieties have differences in their body shapes and fins. For example, ranchu and lion-head goldfish have colorful growths on their head that mane them appear different than other goldfish. The butterfly koi does have a longer tail fin, but other than this, most koi look like colorful carp. If you see a fish with unusual fins or appearance, it is probably a goldfish.
goldfish image by cass from Fotolia.com
Again, both fish come in many different varieties. However, goldfish generally have a wider body shape than koi. Koi have a more torpedo-shaped or narrow body shape. Goldfish bodies are generally more egg-shaped than koi bodies. Be aware, though, that some goldfish do have narrower bodies.
japanese koi carp image by Geoff Hobbs from Fotolia.com
Most of the differences between care for these fish are related to ponds versus aquariums. When goldfish are kept in ponds, or koi kept in large aquariums, these differences decrease. One big difference is that goldfish are small enough to bring inside for the winter if your pond is prone to freezing. If a pond is deep enough, goldfish can be allowed to live in a form of hibernation over a cold winter, but koi ponds are almost always deep enough that this is rarely a concern. Also, some of the very fancy goldfish like bubble-eyes and ranchus must be kept in particular aquariums.