Everything I have been able to find out is that its quite difficult to sex the fish, and nearly impossible if they are young fish. What little I did find suggested that males have more elongated fins when mature, and that females when put infront of a bright light makes it possible to see the ovaries. Other than that I don't think there is much else to go on. If you want to make sure you get some of opposite sex, I'd suggest getting a group of around 6, let them grow up and try to figure out which ones are what, then remove the remaining ones you don't want.