Differences from goldfish

Goldfish 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 on the other hand, were developed from common carp in Japan in the 1820s. Koi are domesticated common carp (Cyprinus carpio) that are culled  for color, they are not a different species and will revert to the original coloration within a few generations if allowed to breed freely.

In general, goldfish tend to be smaller than koi, and have a greater variety of body shapes, and fin and tail configurations. Koi varieties tend to have a common body shape, but have a greater variety of coloration and color patterns. They also have prominent barbels on the lip. Some goldfish varieties, such as the common goldfish, comet goldfish and shubunkin have body shapes and coloration that are similar to koi, and can be difficult to tell apart from koi when immature. Since goldfish and koi were developed from different species of carp, even though they can interbreed, their offspring are sterile.