There are a few blatant problems with the stocking suggested by Charles-n-charge. (Sorry.)
African butterfly fish are territorial and big waste producers. You wouldn't want one in a 20 gallon with small fish, they will eat them (much less two.)
You won't want a chinese algae eater either, they are very territorial and aggressive as they get bigger, and I just refuse to own them. They also get rather big, and wouldn't live forever in a 20 gallon.
Zebra danios do "ok" in tropical temperatures, but are more a cooler water fish, and angelfish NEED tropical temperatures.
Maybe one angelfish, 6 or so corydoras (all the same species, they are happy when kept in a school.) A honey gourami, or a species of apistogramma would probably do well too. Maybe a trio of platy's would do well and are pretty
With such a small tank, you are pretty limited as far as how many fish, but if you are sure to keep where the fish live spread out, it can be a very nice effect.
What do you have in the tank right now? What are you using to cycle it? You really need to be sure to cycle the filter appropriately before adding any fish, since living through the cycling process can cause irreparable damage to the internal systems of fish.