The other replies are right on. You could be down to only one in no time. There are a few things you can do to mitigate this. For instance, adding some dither fish might help, especially if they have similar coloration to the Demasoni. Decorating with rock (I prefer Texas Holey Rock) with a lot of nooks and crannies large enough for hiding will help as well. If you have the space, adding several more Demasoni so that you end up with one male and several females will make the biggest difference, though. They're pretty big before they show their "true colors" and you can tell whether they're male or female without venting. Even venting on these guys I've found to be challenging. If you end up with more than one male, I'm sure you could find someone here who would take him off your hands if he survives. 
