Bite marks on dead fish can have several causes.

Aggression within a species or fish that have been brought together in an aquarium. Study the behavior of the fish and check whether or not aggression is an issue. In a multiple company aquarium also the composition of the company can be an issue. Even members of a similar species have to be observed and analysed.

When the fish do not show signs of aggression amongst each other, one can assume that the dead fish has already been in the aquarium for several hours and simply has been eaten by the other fish. In this case, watch the fish closely for the next few days in case any signs of illness come up. If the dead fish has been ill it could have infected the others.