If a shark ate the fish, then, yes, the fish got eaten by a shark. Past tense.
However, "gets" can be ok, too. Maybe you are talking about what happens when a fish is near a shark. If the shark is hungry, then what usually happens is that the fish gets eaten by the shark. Present tense.
However, since "fish" can also be plural, it may also be the case that the fish get eaten by the shark.
Ain't English great?