The answer is, it is both.
We usually use “fish” as the plural of fish when we are talking about 1 kind of fish.
I saw many spotted fish today in the lake. (They were all the same kind of fish.)
We usually use “fishes” as the plural of fish when we are talking about 2 different kind of fish.
I bought 10 different kind of fishes for my fish tank. (They were all different species.)
However, according to most dictionaries you can use either form in either instance.