What is the plural form of "fish"?
It honestly all depends on the context.
The most common (plural) form of "fish", at least in the United States, is "fish". We do use "fishes" in some instances although "fishes" isn't as common. Let's take a look at a few sentences so that I can elaborate:
"There are four fish in the tank."
In the sentence above we are saying that more than one fish is present in the tank (in fact there are 4 fish).
If we are not specifying a particular species and are only commenting on the number of individual fish, we use the term "fish".
If we are commenting on a number of different species of fish we can say:
"I have two different fishes in my tank, 2 puffer fish and 2 clownfish."
In the above sentence by using the word "fishes" we are stating that there are multiple species of fish, not that we have multiple fish (or more than one individual fish).
In fact (to make things a bit confusing) you could even rewrite the sentence above using "fish" by saying:
"I have two species of fish in my tank, 2 puffer fish and 2 clownfish."
It's just a matter of preference (when referring to multiple species), I'm privy to, and almost always use "fish" in my speech and writing because "fishes" sounds slightly awkward.