I would switch up the aquascape in your 55 gal
. Be creative make additional caves and hiding places. Arrange the decor as to break up the line of sight, then reintroduce the troublesome fish. I keep New World and Africans cichlids together in several different tanks. its not easy and few recommend trying it. I initially used the Africans as dither fish for my large S.A. cichlids, but now consider them just as valuable. Mbuna are true fin splitters. Good luck.