Pretty much anything that's not aggressive or small enough to be eaten (and you'll be AMAZED at how large a fish that leaf fish can eat). Leaf Fish of all sorts (South American like Monocirrhus and Polycentropsis or African like Polycentrus or Afronandus) are relatively shy, unaggressive and not particularly active fish. They will not defend themselves against tough, active fish like cichlids, etc.
The MAJOR drawback to pretty much all of the leaf fish (especially Monocirrhus which is probably the one you have) is that they often won't eat anything but smaller live fish. Do you know which one you purchased? Any pictures, perhaps?
-Joe