Several. How big of a tank are we talking about? You could go with porcelain crabs or pom pom crabs if you have a smaller tank. They tend to get lost in bigger tanks because they usually only come out at night or when feeding. If the tank is bigger, green/red/black mithrax crabs or sally light foot crabs. Some people say they will grab small fish when they get big enough, but just like most other information, it's probably second hand from someone who claims they saw their green mithrax eating a fish. Chances are, that fish was already dead and the crab is just doing it's job of cleaning up.