I've read the the the males have pointed tips on either their pectoral or anal fins, and the females have more rounded tips. I've looked at 20 or so that have come through my LFS, and they all have points. I got a smaller one and added it in with my larger one that i've had for a year or so. They've been getting along well for the last 3 weeks, so I think maybe it's a pair. Good Luck, there is some pretty good info here if you search hard enough for it. I believe they will change sex and form a pair if purchased young enough, but again I'm not completely sure.