For 30 gallons, I'd go with not more than one mated pair, however I know people who keep two. They also will appreciate gravel and driftwood in the tank. I have one mated pair in a 45 gallon, and they will not let anyone but the fry in there. Even harrased, almost to death, plecos and synodontis catfish. Be forewarned, that a mated pair will be extrememly agressive. Other tankmates would include larger cichlids, and your tank isn't well suited for them.