Have you done saltwater before?
If not, do not just put fish in a 20 gallon without doing a lot of research first.
A 20 gallon is very small for saltwater fish and therefore should only be used for clownfish or other very small reef fish. No Puffers, Triggers, Angels etc.
A pair of Percula Clowns would probably do okay if you have the proper filteration, and do weekly water changes. Add a lot of Live Rock and you can do corals to add to the tank.
Invertabrates will be okay too. Hermit Crabs, Shrimp etc.
A 20 gallon tank is in some ways more difficult to maintain than a large tank, so research everything far in advance. It takes patience.
Good luck