I had about 15 Mubna (mostly adults) in a 55g for about 2 years. It was not ideal by any means, but they were healthy and no one got murdered during thing night. I do agree with Blair though, knowing what I know now I would not do anything smaller than a 75g for ideal conditions for mbuna, but if you are forced to downsize then a 48" 55 is "doable". I would not suggest a 40" tank though, there's just no room for escape and someone will for sure end up dead every few weeks.