Well, the safe rule is an inch of fish per gallon, so needless to say you have room for addition. What additions? obviously community fish are a good idea, probably some variety of larger tetras or livebearers like mollies, platies, or swords, probably avoiding anything with flashy fins as the barbs will nip at them. Be sure not to add more than three or four fish at a time either as it may overload the filter making the water deadly for the fish! good luck!