Hello! As far as I remember, the singualr is a fish, the plural is fish, but fishes mean different species of fish. For example,  two fish means  two fish of the same species, but  two fishes mean  two different species of fish. This rule is not only for the word fish. The same is for fruit and fruits and other uncountable words if I am not mistaken.

I hope I was helpful.

Your first option for the plural should almost always be "fish", eg. two fish, eight fish, plenty of fish in the sea.

There's an interesting reason why we have this form (one fish, two fish), and it also explains some of our other irregular plurals, such as mice, geese and men.

Natalia's right about using "fishes" to mean "different kinds of fish", but it's very unlikely you'd need to do this in practice. Even at a fishmonger's, you'd say "there's a lot of fish".

"Fishes" also turns up in Biblical texts, so using "fishes" sounds a little poetic.

Hello.  Fish is a noun and a verb. Singular and   Plural noun is always fish, I believe.  "There are plenty of fish in the sea."  But the verb "to fish" can be:  "He fishes with his son."  

Hope that helps. 

The noun 'fish' is uncountable.  1 fish, 2 fish.

People who say two fishes are making an error. You'll often hear children do this. 

You may hear it on TV when the actor is pretending to be someone who is not well educated (didn't finish highschool). For example, a common sentence in an old gangster (Mafia) movie might be: 'He's sleeping with the fishes.' However, most native English speakers would know that that sentence is 'wrong.'

Usually the saying is: There are many fish in the sea.  [meaning, you can find a new boyfriend/girlfriend]