Diet in the wild consists of small fish.
The aquarium should have some rockwork but still provide lot's of open swimming space both along the bottom and mid-tank region. Feed a quality cichlid flake/pellet food, supplement occasionally with frozen brine, krill, mysis and/or feeder fish.
Rhamphochromis are predators and will consume smaller fish. For that reason, they should not be kept with smaller Haps and/or Mbuna.