In a 20 you are limited to a pair of some small cichlid, dwarfs would be a good choice. What are your water parameters like?
If you water is on the soft side i would go with a pair of dwarf cichlids like apisto.caucatuoides, rams or kribs
If your water is hard (7.8 +) then i would try some shell dwellers from lake Tanganyika.
These are jsut a few examples. What are you interested in?