Well... AA master
You may have to stake the tank out for a while to catch who's doing it. I had a few issues and had to watch my tank for hours at a time a night to see who the bully was. It turned out one of my Mbuna males had turned "hyper dominate" I quickly took care of him and caught him and brought him to my buddies fish store. He put the fish in his display tank and he ain't so tough now.
Look for signs of your fish staying away from one fish. I mean you'll notice it. Example, I'd look at my tank and notice all my fish pretty munch stayed on one side while the bully stayed on the other side or went where ever he wanted to and they all just ran away. Since removed the tank has settled down.
I think you have your homework cut out for you. Good luck.