@Verity but fishes sounds soooooo much cooler.

Anyway.
"Fish" is one of those odd words that's usually both singular and plural—like "deer" and (in Canada) "beer." So we can say:

We caught five fish.
We saw three deer.
We Canadians drank six beer.

But we can also use "fishes" when we are talking about different species:

The fishes of the Fraser River include salmon and sturgeon.

@Verity but fishes sounds soooooo much cooler.

Anyway.
"Fish" is one of those odd words that's usually both singular and plural—like "deer" and (in Canada) "beer." So we can say:

We caught five fish.
We saw three deer.
We Canadians drank six beer.

But we can also use "fishes" when we are talking about different species:

The fishes of the Fraser River include salmon and sturgeon.