They would eat a very small fish if given the opportunity, but the fish would have to be very small (to fit in it's mouth) and in very bad shape or dead. Tubastrea really isn't as aggressive as many of it's NPS counterparts, and even a coral like a Rhizotrochus has a tough time catching a healthy fish. They'd have to make the mistake of swimming directly into it's grasp.
I've never heard of any Tubastrea sp. eating a healthy fish in an aquarium.