[letney]

"fishes" refers to multiple species of fish, while "fish" is the correct plural form of a school of a single species.

[MrJagil]

Awesome!Though, which one should you use if both are correct?

"... plenty of fish(es) in the sea." for instance?

[vidarh]

That distinction is likely to die out, given that lots of other English words that used to predominantly or only use the same form in singular and plural are steadily moving towards separate plural form.