Moonlights get to 6" easily. Due to this size and the fact that males can be aggressive/territorial, I would personaly only keep one for a community. Otherwise, you could keep a trio quite comfortably provided there aren't too many other fish - particularly inhabiting the upper water layers - and they have plenty of cover. By trio I mean a single male with 2 females. If you have 2 males (or more), you are asking for trouble - especialy in a tank with limmited upper layer territory space; such as a 30 gallon. Males can quite easily be distinguished by their longer and more pointed dorsal fin. The females are also wider, when viewed from above, and deeper-bodied. When in breeding condition, males have distinctly more orange ventral fins than those of the female which remain yellow.