Those are some big emerald crabs! If they've cleaned up what you needed cleaned up, it might not be a bad idea at all to trade them. I don't think you'll have to worry about the eating your fish. If you know anyone that needs one or both of them, you can probably get something decent in return as a trade, considering emerald crabs eating Valonia doesn't seem to be such a sure thing and you have some that you know will eat it (I'm guessing that's why you got them in the first place).