For some nouns, like fish, there is no difference between the singular form and the plural form. For example one pen and two 'pens'. But sometimes the noun doesn't change for example one fish and two fish, also one sheep and two/many sheep. One furniture and two or more furniture. Here noun doesn't change in plural form. This is the rule. And fishes mean different species of fish. To make it simpler I give you two examples- There are many fish in the river ( here you are talking about one specie of fish ) There are many fishes in the sea ( means fish of different species like piranha, jelly fish, star fish)

The word “fish” can be used as either a mass noun (“how much fish did you eat?”) or a countable noun (“a lot of fish are swimming around” / “Joe ate a whole fish”). You are correct that it is a plural countable noun in your example. The thing is, “fish” usually has the same form in the plural is in the singular — this is a legacy of Old English. The only time we say “fishes” is to refer to different *kinds* of fish, and even then, it tends to be optional. By the way, you’ll get more help if you use a more descriptive title. How about “Fish or Fishes?”

You can see a short list of when single and plural do not change, here in the first list at the top.. Note they are mostly animals. There are others but this will get you started. https://www.theenglishspace.com/grammar/nouns/irregular-plural-list.html