Their cells can be red, yellow, black, white, blue ... and is the most expensive koi fish ever sold. Koi may just look like an oversized goldfish, but they're not even remotely related.

"The popular koi fish of today are actually ornamental varietals of domesticated carp," Hilstrom says. "Black carp originate from China and were first bred for red color mutations (noted around ...

Koi carp are one of the most iconic ornamental fish in the world. Originally hailing from Japan, these beautiful fish have amazingly patterned bodies. There are many varieties–– and some of them are ...