Depends on what type of fish. Nuclear power plants used to dump their cooling tanks' water into rivers until they discovered it was massacring all of the fish in the river because warm water holds a lot less dissolved oxygen. But some fish are more suitably adapted to warm waters.

depends on what fish

Where did you hear that? My fish tank is kept at 68 degrees F for my freshwater fish.
If the temp goes around 85+ and the water boils then you got sushi right there.
Certain fish need certain temperatures, so maybe you're thinking of fish that live in naturally cold waters?