I agree with kuhli, get 5 or 6 in a 20. Is it a 20 long or high? 20 long will open up your options a lot since it has more space on the bottom.
My tanks (so far
5.5 gallon Planted Dwarf Puffer tank (2 Dwarf Puffers and a lot of RCS!)
16 Gallon Planted
(GBR pair, 3 amano shrimp and a whole bunch of snails!)