75G would be close to 300 liters or so.
but Mbuna must mean you have to feed a fairly veggie diet to keep those healthy and also, Mbuna are boisterous and often agressive.
true njassae wouldn't like a tank like that, but unfortunately -given the sheer rarity of true njassae- I can safely assume you have either S occelifer or so, or a hybrid.
added Syno's should cope with the above (green diet, agressive boisterous cichlids). two species from Tanganyika are highly suitable, and both are shoalers. wich means 5 or so makes a nice group; 5
of the same species.
more is better, but with a total of 7 Syno's it's nicely filled.
the 2 species are
Synodontis petricola, wich has a lake-wide distribution and comes in at least 3 varieties (the very common Zambian "dwarf" wich is bred in captivity in huge numbers; the rare true petricola from Burundi ea and the huge and expensive "petricola sp Giant" from Tanzania)
and
Synodontis sp "polli White", wich originates in Zambia and is also bred in captivity; mostly sold as "Synodontis polli". the true S polli is a big brownish spotted beast with a huge, wide mouth, not a pretty contrasting black and white fishy.
both these are quite omnivorous and hence, cope with veggie food like OSI spirulina flake or Wardley's for example. good Mbuna flakes
Also, all Tanganyikan Syno's are -of course- used to dense cichlid populations.