I agree with CFC, that is too small for most cihclids. Especially as the tank is tall, that leaves less territory. I don't know much about mbuna but I would never put them in a 20gallon tall, they are very aggressive and need a fairy large, overstocked tank to keep them happy.
You could get a pair of rams in a tank that size I think, some people keep them in a 10gallon but I am not sure if that's a good idea.
HTH