What is the plural for fish if it doesn't end in s?

The spelling 'fish' can be both singular and the plural (One fish, two fish, red fish, blue fish... to quote Dr. Seuss).

Examples:

Plural: Look at all the fish.

Singular: I caught one fish.

But the plural fishes can be used when referring to more than one type of fish.

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For types of fish, the same applies, where the plural of trout may be trout or trouts, and the plural of bass may be bass or basses.