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2 oscars in a 75g is the barebones smallest tank size. Those 2 fish will foul your water quality quickly on their own. I kept my breeding pair in a 75g while they were laying eggs and it took 2 Fluval 405's with 2 50% weekly water changes to keep the nitrates at 20ppm. I'm not saying it can't be done by between water quality and aggression I would dump the mbuna and concentrate on the oscars and their heath or look for a larger tank.