"the conditions of Niigata in the mountains are perfect for raising koi, all the mud ponds they are raised in are filled from snow melting up the mountains."
Same arguments are, or were, made about bonsai. The soil in Japan is/was supposed to be VASTLY superior to anything else worldwide; Westerners were incapable of "understanding" bonsai and therefor any we made were immediately inferior; Only the Japanese understood what bonsai was about. Yadda, Yadda, Yadda
All that is simple claptrap, bred by the over clubby atmoshpere and a twist of Xenophobia in Japanese bonsai circles. In the last 15 years, or so, those arguments have been seen, for the most part, for the BS they are--even by the Japanese. The west figured bonsai out--it wasn't the soil, but how the soil worked that made the difference. We adapted with crushed pumice, haydite, granite and other stuff that performed like the vaunted Japanese soils. Westerners honed their perception of bonsai, perhaps even more so than the Japanese, as at it's worst Japanese bonsai can be simple rote imitation of past --piles of green doughnuts on a trunk--The more naturalistic and/or collected trees being worked in the west show a dimension that the Japanese have ignored.
As for Koi, I smell the same thing- a clubby atmosphere that doesn't want to let the outside in--Snow melt is snow melt in Japan, North America, Europe, where ever...