I have two Green Chromis in my 55 and they're peaceful towards other fish, BUT I've seen them picking on other fish when they're sick. They pick the heck out of my Sailfan Tang when she was sick and cost her life. They nipped her until she dies, they did that when I wasn't home, it made me mad. That was quite sometime ago when all that happened. But if the fish is healthy, they generally leave them alone. Damsels are aggressive!!!!! I still have the Green Chromis though. They're not picking on any fish right now since all my fish are healthy.