A 20g tank dramatically limits what you can put in it, as it's too small for many single fish. Additionally, other species which, by themselves, would fit in a 20g, won't be healthy or happy in less than a school of 4-6, which might be too much for a 20g.
Nah man, there are TONS you can do in a 20. Just not a lot of room for variety. Tetras, rasboras, dwarf cichlids, cories, barbs, livebearers, list goes on and on.