"Fishes" exists but it tends to be literary, scientific, or archaic. While it is a word, the plural of fish is, nine times out of ten, fish.

Fish is countable and uncountable, we use it for the singular and plural form. Do not say fishes.

"Fish" is the plural form of "fish". It doesn't change. This is quite different to it being an uncountable noun. "Fish" as an uncountable noun means "fish meat". "Fishes" suggests "different types of fish". However, as Kevin and LGF92 point out, you really wouldn't use this.

As others said, "fish" has countable and uncountable meanings, but 99% of the time the plural form is still "fish." Countable: There are 10 FISH in the aquarium. (NOT fishes) Uncountable: I ate a lot of FISH in Japan. He bought a kilo of FISH at the supermarket. "Fishes" also exists, but it would be incorrect in sentences like the ones above. It is used in scientific contexts to talk about different species of fish. The fishes of the Atlantic include salmon and cod. Remember that this use of "fishes" is very rare, especially in spoken English.

Fish is countable noun, so fishes is right!