Plenty of cichlids which could live in a 30 gallon. All of the south American dwarfs like rams, apistogramma, even dwarf crenicichla, would work. In a pair or perhaps a 1m 2-3f group depending on the species. From Central America, amatitlania nanoluteus pair would be great.
If you want something a little bigger and more aggressive, could probably keep a convict or male of any of the amatitlania group solo. Though, a 40-50 would be better for a big male convict after a while. A female of any of these species would be fine.