If you're talking about plural types of fish without using "types" as a modifier, then I think it's appropriate to use the plural form. Or can be. I typically use "fish," seeing it as having similar rules to "deer."
So if I was describing many of one kind of fish, I would say, "Look at all the fish!"
But if there was a gamut of different kinds, it wouldn't bother me if somebody said, "Look at all the different fishes!"
About the only time you'll hear me use "fishes," though, to be honest, is the verb form. I fish. He fishes.