There are a ton of cool fish you could keep in a 20 tank.
How about dwarf puffer fish? They only get 1 inch.
Dwarf Gouramis are pretty cool too. They have amazing colours.
There are also cool amazon dwarf frogs, or fiddler crabs. Or salamanders.
For most freshwater fish it's about 1 inch of fish per gallon.
I have a salt water nano reef in a 20 long tank. It has a bunch of crabs, snails, shrimp, starfish, coral and other really cool things.