is it a 20 long or 20 tall?
if it is a 20 tall i would do...
-School of 6-8 Zebra Danios
-2 Coolie Loach
-Pair of Dwarf or Sunset Gourami
if 20 long...
-Pair of Dwarf Cichlids (Rams or Apistos)
-School 8-10 Neon or Cardinal Tetra
both tank versions would be planted