Koi, an ornamental fish which enthusiasts liken to a moving work of art, are gaining popularity thanks to changing lifestyles and increasingly sophisticated tastes, experts say.
Koi swimming slowly and gracefully in the water provide self-healing and peaceful feelings to a person, and koi never fight among themselves.
Most everyone involved in this hobby refer to the Nishikigoi as just Koi. So, when you find information relating to Koi in Japan dating back to 200 BC that is when the invading Chinese brought the common black carp to Japan. The statement “Contrary to common belief Koi did not originate in Japan” is true if you are referring to the common carp but not true when referring to Nishikigoi.
A grand champion could fetch 10-15 million yen (95,000-143,000 US dollars) at current market prices, or even more. Europe and the United States are the world's biggest buyers of koi in terms of volume, but the most expensive variety is sold in Asia because Asians have a better appreciation of their value.
( and everyone else who breed them )
But you can read more at the sites I have included in this post.