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fish, fishes

Fish is the preferred plural.

Fishes may be used in scientific articles when several species are meant.

FishBase is a reliable database of common and scientific names.

Many common names ending in "fish" are written as one word:
anemonefish
angelfish
baitfish
batfish
butterflyfish
cardinalfish
damselfish
flyingfish
hogfish
lanternfish
lionfish
lizardfish
parrotfish
rabbitfish
scorpionfish
squirrelfish
stonefish
trumpetfish
unicornfish