Fish has two possible plural forms

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The word fish is a little special in English because it has two possible plural forms.

Regular Case

Singular: fish

Plural: fish (Used when talking about multiple fish of the same species.)

Example: I caught one fish yesterday. He caught five fish in the river.

Alternative Plural: fishes

Plural: fishes (Used when talking about different species (kinds) of fish.)

Example: The aquarium has many fishes (goldfish, angelfish, and catfish). Biologists study the various fishes of the ocean.

Summary: Fish - Fish (general plural, same kind), Fish - Fishes (scientific / species plural, different kinds)

Everyday English usually uses fish for plural.Fishes is more formal or scientific.