Males tend to be a brighter yellow with darker black fins. They sometimes have egg spots. The best and 100 percent accurate way to sex them is to vent them. Take them out of the water and look at their anus. If there is a while hole that seems fairly large (for letting eggs out), it's a female. It it seems pretty small, then it's male. I keep 5 in a 29 G. Many people do not recommend it. I have 1 male and 4 females, as males can be very aggressive, if you do use a 29, keeps more than 2 females in there, and get a lot of rock work,