Snorvich is right, they will commit suicide trying to eat fish they can't swallow. They are also liable to overeat until they die if you put too many food fish in the tank. That said, they aren't particularly good at chasing down fast fish like damsels, and if you don't feed them something that is pretty slow you can even starve them to death. Despite all the above, I've found them to be finicky eaters. They are beautiful tank residents, though.
Cheers,
Don