If you are talking about more than one fish of the same species it's "fish" if
you are talking about more than one species of fish then it's "fishes"
So All these sentences are valid:
The fish is fresh.
The fish are fresh.
The fishes are fresh.
The fishmonger's fish is fresh.
The fishmonger's fish are fresh.
The fishmonger's fishes are fresh.
He sells fresh fishes.
He sells fresh fish.
/Sameer