The cold hard facts are a 20g is very small
for a saltwater system and you very limited
in the fish you can SUCCESSFULLY
keep in there. Obviously you can try whatever you want, but a 20g is a 20g and it has it's limits.
Do some research on the fish you may want to keep. This site is very good, but keep in mind the minimum size tank recomendations are the size tank the fish should be in when it's young and still small.
Saltwater Fish: Marine Aquarium Fish for Saltwater Aquariums