The Melanochromis Johannii are highly aggressive and IMO belong in a tank no smaller that 75g. Not to mention he will probably end up killing all your other fish. I suggest removing him from that tank.
Here's some info on the Johanni...
http://www.timstropicals.com/Inventory/African/JohanniInfo.asp
Here's some info on the Johanni...
http://www.timstropicals.com/Inventory/African/JohanniInfo.asp