I tried, and failed, to keep mbuna in a 30 long. The "winner" or "survivor" or whatever you want to call her is living out her days in a 10 gallon tank in solitary confinement. I've had her like that for a while now, and although she gets bored an rearranges the tank from time to time she is healthy and happy. The tank also looks good with just the one fish (Labidochromis caeruleus) in there.