I have 2 in a 50g, I believe they're both females since i've seen them both w/ eggs and there's none other than those two. I'm guessing they just produce unfertilized eggs? Anyway, when they see eachother, the bigger one wants to beat up the littler one, but there's enough room for them to coexist. If you can tell what sex you have and try to get the opposite, it'd probably work better since it looks like you have nanos.