Fish and water temperature

What makes tropical fish tropical in that they die in cool water?
Why can goldfish live under ice while their relatives koi cannot?

Bump, gosh darn it.

Koi live just fine under the ice as long as there is still unfrozen water under the ice. You do have to watch out for a toxic buildup of harmful gases that can acumulate under the ice so it is best to try to keep a hole open for them.
mind you I am talking about a backyard pond which is what I think you were refering to

Well, yeah, but then they aren’t living under ice. Goldfish do, and why?

Oops, sorry. I misread your post to be “Water not frozen over”.
The Aquarium fish magazine guy, Meyer, says Koi can’t live under a frozen surface.

Again, I apologize.
So why do tropicals die at 50F?