Just Edits

Is the noun fishes the plural of fish, and if it is, why do I so rarely hear anyone use the word?

Well, yes, fishes is the plural of fish—sometimes. But fish is also one of those nouns with singular and plural forms that are exactly the same—like deer, moose, and sheep. 

So, when should you use the word fishes?

When referring to multiple species.
Example:
Of all the fishes in the water, the students studied the flounder.

When using the plural possessive for fish.
Example:
The fishes’ colors were beautiful. 

When using a verb.
Example:
He fishes in the Suwannee River every weekend.  

In all other cases, like when referring to a bunch of the same fish, use the word fish.