The plural of fish is fish. However if you want to refer to multiple species of fish then you can use fishes.

Its "You've got so many fish".

Both are correct

Each phrase is correct, the meaning depends on

Both: "You've got so many fish." (perhaps they are all the same species? it's not clear) "You've got so many fishes." (there are different varieties/species of fish) Same as other similar plurals: "I ate lots of cheese last night." (one type of cheese? many different types of cheese?) "I ate eight different cheeses last night." (clearly different types of cheese) meat/meats, wine/wines, beer/beers, etc But to be fair, "You've got so many fishes." is perhaps not very common (in my opinion). I think people normally prefer to construct a sentence like "You've got so many different types/varieties/species of fish." because we think that makes things clearer.