The crushed coral will also slowly dissolve and help provide minerals for the water. No matter what sort of filter combination you decide upon, do perform frequent partial water changes. Remember to keep using the special additives that Lake Malawi cichlids require. Though not as important as with salt water reef tank, a tank of Lake Malawi fish should be brightly lit. You want the best lighting possible in order to enjoy the awesome colors of these fish.
Light is important for another reason.
Many of the mbuna are basically vegetarians. It is a good idea to allow as much algae to grow as possible. Many good vegetable foods, some designed just for Lake Malawi cichlids, are on the market. My guys seem to prefer the ones that contain spirulina. Malawi cichlids are not fussy.