I had 2 large emerald crabs.. Had is the word here. I watched one sneak up on my Ruby Red scooter male and rip it's left fin off.
I destroyed my aquascaping getting to him, but he came out with a piece of rock and I didn't realize it until my husband was screaming like a little girl that he nearly walked on it in the kitchen.. So, I flushed that one, caught the other one and the half black angel (it was eating my wellsos and Acans), and they are no longer living in any of my tanks.
I still giggle over the screaming from my husband. Lol
But yeah..they do eat living fish. Since the crabs have been gone, my dottback, firefish, and Anthias have all come out of hiding.