David wrote:

The next obvious (and reasonable) question would be, "what happens
when all these tropical fish are subjected to freezing temperatures?"
Well, suppose this affair was to be installed in S.E. Asia, where it
never freezes. (Which if this thinking works out, will be the case.)

They die. That is why they (tropicals) live where it don't freeze.

Temperate fish live where it freezes.

You could stock something like sunfish, or bass, catfish, pike, muskies,
etc, in addition to your koi, assuming they (koi) are big enough not to
be eaten.