They eat everything that moves. Doesn't matter if it is bigger than they are, or if it is posionous/venomous. I had a 4" frogfish eat a 5.5" foxface....you would have thought I would have learned after that, but the same frogfish later ate a 6.5-7" lionfish. I kept looking around on the floor and under all the rocks trying to find that lionfish. I kept thinking.... no way is that possible!
On a care note please keep in mind they cannot come out of the water at all, if they swallow air they will die. They should be kept in a dedicated system, if kept with corals you should know that they will pearch on them and might irritate them. You shouldn't even keep them with fish that are drastically too big to eat them because the other fish might pick on their lure... or you might find out that they were not as big as you thought...:mad2: