Well, I have 6 in a 135, it turns out they are three pairs and they get along, as long as everyone minds their p's and q's. I don't know how you will sex them, there are no external sex differences that I am aware of, In the book - South American Earth Eaters" they state there is no sexual dimorphism. And I can only tell if they are spawning, and then it only lasts a few days, and then I can't tell. They are housed with 3 large (10 - 11 inch) Uaru, 8 smaller jurupari and a host of somewhat fancy plecos. The Altifrons are around 8 inches now, not including very long fin extentions.