I wouldn't get anything large. You don't really want to put anything over 5-6" in a 30. Fish grow fast, and it's generally a REALLY bad idea to stock fish that will REQUIRE an upgrade in a few months. If you're THAT dedicated, THAT ready to upgrade, then do it now. If you're getting a fish that needs a 200-400+ gallon tank (such as the arowana), get a 200-400 gallon tank. Otherwise, settle with the selection of interesting fish that fit in your 30, such as senegal bichirs, various tetras, small knifes (african brown knife, centipede knife), leopard ctenopoma, african butterfly fish, shelldweller cichlids, etc etc etc.
