Well, as people above have mentioned, there are a lot of options. 20g long is prettty flexible. You could keep most small fish (under 4"). The key is what combination of fishes you choose.
If you'd like a nice, peacefully community tank, here are some ideas for you:
Top dwellers (choose 1):
6 danios (zebra, pearl)
3-4 livebearers (guppies, platys, swordtails)
1-2 dwarf gourami
Middle dwellers (choose 1):
6+ rasboras (harlequins, red-line, etc.)
6+ tetra (many options)
2+ cherry barbs
Bottom dwellers (choose 1):
3+ cory catfish (many varieties)
3+ otocinclus
3+ loaches (smaller varieties only, no clowns)
1 Siamese Algae Eater (could be in addition to above choice)
One other option might be 1 or 2 of the more peaceful dwarf cichlids like Apistogramma or Kribensis.
There are a lot of things to take into account, but most of the above species would get along. Do a little reading and let us know which fish catch your eye. We'll be happy to help you make sure there aren't any compatability/overstocking issues.
HTH!
