How old are your tanks? Have they been fully cycled? What kind of filitration are you using?
W/ african cichlids you want to over do the filter systems for example we are using a
canister filter made for a 60 gallon tank in our 55 + a penguin biowheel made for a 75... I also prob over do it but I do water changes very often. They have huge bioloads, and your 6 in a 30 is not a bad number however, I would make sure you have proper filitration, and are testing the water. If your tank is not cycled because its a new tank, then that can have a lot to do w/ a sick fish.