i have a 20 gal. as well. in it i have a blue damsel, a per. clown, and a clarkii clown, along with 2 hatian sea anemomes. the best thing to do once you get about 3 fish in there is READ about FO tanks and Reef tanks. get some clowns, chromisis and maybe one damsel. the damsel might become a little agresseive and territorial, but my clown and damsel seem to like each other and they swim together all the time. but the best thing for you to do is read after you get the fish. dont get to many fish though and have fun! this is great hobby to get into.