This happens esp. when you are dealing with Malawi mbuna. It sounds like they were stressed and that Mbamba took over dominance when you added him in. He probably picked on the labs. Labs are probably one of the more laid back of the Malawi so when you add new fish you want to make sure that the aggression level is relatively the same. The Mbamba sounds like it was just to aggressive for them. I had a constant balancing act when I had my Malawi tank. Its kind of like finding an equilibrium to balance out the aggression. In honesty having all males was probably a better choice because if they have females to fight over, they will. At the same time if you get enough females per one male that can also help with the aggression. I know when I would add fish I would always lose fish until it was back to the same number that was in there before. Its like this one number of fish such as 5 fish was the key to them equaling out. Sometimes it works out and sometimes it does not. Sometimes fish that are normally less aggressive can be more aggressive and vice versa you just have to keep working at it and researching which fish are most compatible together and eventually you will find a group that works out together. Hope everything works out with the rest of your fish.
