I was thinking of a dwarf cichlid. I am fairly new to this but I have had a 75 gal with a colony of demasonis in it for about a year and a half now. I am wanting to do another tank now. I am currently looking for a 75 or higher but no luck. I do not know what other dwarfs are out there so that would be another question that I need help with.
Among the Malawi, even the dwarfs need a bigger tank than 20G. Try the same post in the Tanganyikan section, there are some Tangs that can be kept in a 20G (but not too many since it is a 20 High which has a smaller footprint).