I've had luck with a "time out" just like for kids it's also works with fish. I've used the main tank or another tank works better, but just like with junior remove the aggressor and either leave him netted in the main tank or better is move him to a smaller tank for 30-40 minutes. This has worked numerous times with aggressive mbuna, so much in fact it's changed dominance from fish to fish. There are times when it's taken more than a half an hour, like overnight ( overnight is not for fish in a net ) but that's been rare.