The one I had was pretty aggressive towards other fish that were it's size or smaller. It actually killed a Blue Damsel by chasing it relentlessly around the tank. It was also aggressive towards my peppermint shrimp, and was also implicated in its demise. I am not sure if they are all this way, but I think they tend to be pretty territorial if they have the opportunity. I don't know how they would treat corals, my guess is that since they are not a threat it would not bother them. That said I still think it is a cool fish, just be aware of its personality.