Mbuna are vegetarians and can not properly digest fleshy foods so only feed them vegatable based foods. You can always suppliment their normal diet with cucumber and garden peas. Buy any cheap bag of frozen peas, leave a few out to defrost or place them in some boiling water for a few minutes. Once defrosted split and remove the shells then drop the peas into the tank.
Peas are always appriciated by any type of Mbuna and can be refrozen without the shells and then defrosted for use at a later date.