I can't imagine an emerald crab catching fish. i have had probably 50 emeralds since I have been keeping reef tanks. Though they will eat about anything have never seen one harm a fish or other inverts. Humu Trgger is aggresive and will eat anything it can corner. Tusk fish are known to eat invertebrates such as shrimp and crabs. I don't know how big your fish are, but I would bet 1 if not both are your culprits.