What crab lives in a shell of other fish?

There is no known crab species that lives in a shell of other fish. Crabs typically use shells of mollusks, like snails, as their homes.

It's possible you're thinking of a different creature, perhaps a hermit crab. Hermit crabs are known to use a variety of empty shells for protection, including those of snails, clams, and sometimes even other crustaceans.

Could you clarify what you're thinking of?