If you are looking at tangs, you have a lot of options. 20g is a perfect single species tang tank. Here are some species tanks that can work:
Any Brichardi complex fish: N. Brichardi, N. Pulcher, N. Helianthus, etc. Start with handful of juvies and keep the first pair - you will soon have a colony.
Any smaller Neolamprologus, like N. Leleupi, one pair. Be careful with leleupi, 20g doesn't leave much room for error, if your male is getting tough, put in a divider asap.
Just about any Juliochromis should work. A pair or a trio
Altolamprologus Calvus, a pair or a trio.
Shellies, either 2-3 pairs or a colony depending on what species.
Here is a link to another forum with a "20g cookie-cutter" page.
http://www.cichlid-forum.com/articles/cookie_cutter_20g.php
Livebearers, killifish, gouramis, paradise fish are all great things for a 20g. A pair of golden panchax or a group of Xiphophorus montezumae will make for an awesome tank.
There is a "mini monsterfish" forum on mfk, I think one of stickies goes through many cool fish that can live in small tanks, like leaf fish and mini payaras.
You can also try puffers. You could get a few pea puffers in that tank, or maybe even a pair of figure eights.
Unless you are an experienced aquariast, I would stay away from ram cichlids, though. They require pristine water quality and really need RO water, plants and huge WC and a big tank for stability. People recommend them for small tanks due to their size, but they are one of the hardest fish to keep.