You will not be able to breed neon tetras with a breeding net in the main tank, they need weeks of conditioning, a special spawning setup with mops and specially treated rainwater, and then the fry need special treatment and raised on rotifers. Unless you have a spare 15 gallon tank, spawning mops, a spare heater and a spare filter, you might be better off trying to breed livebearers. Neons are extremely hard to breed because when they reach their full size (the size they are sold at) they are already past breeding age, you need to get juveniles to attempt breeding. Most people who try to breed neons and fail are trying to breed with fish that are already too old. It's hard to sex neons, the only thing to go on is that females are wider and get fat when well fed, but an overfed male will get fat as well. There is a calculator at the top of the forum for working out the capacity by using the dimensions. I think it is large enough to add a few more fish (I THINK). If you want babies, try platies. They breed really easily with no help from aquarists. And raising hte fry is fun because they are so cute!