aggressive

I have 4 cichlids in a 90 gallon tank with plenty of space and caves/hide holes. 3 of them get along and one is a total jerk. He is picking on one that is injured (I bought him that way) and I noticed a piece of fin missing on my albino oscar. I am not sure what else to do. I keep catching g him...

So i think i found out nearly for sure why i keep losing fish seemingly randomly. My pearl gourami was just going on a rampage, chasing everyone in the tank nonstop, bashing into all of them... they were all so scared and banged up from him that they all hid in the plants and refused to come...