I would take them back if you do not have another tank. Mbuna need a veggie diet to avoid bloat, no question about it.
There is nothing you can do to your tank to prevent the inevitable, it is just their nature. African cichlids, for the most part, require their own tank, often overstocked to prevent aggression, and they are not good community tank members. If you like cichlids I think keyholes or some angels would be very appropriate for your tank.