Lots of food and pristine water conditions are the key to good growth. I just got home from a livebearer convention, yes there are such things, and one of our speakers was a commercial breeder. He basically said that as long as you can keep food always available and keep the water pristine, you can expect near optimum growth. He even cited some huge fish that he keeps in rather small confines, far smaller than any of us here would consider acceptable, where they keep growing very well because they are always well fed and always get fresh water continuously. In his case he was talking about catfish that are over 3 feet long that are living in a container so small that each must move out of the way for the next one to reach their food. Now that is very tightly packed, but they keep growing because they are living in his commercial filter of a few hundred gallons capacity and they get a constant supply of fresh water from his various growth tanks, not just the filter tank's water. None of us has the ability to provide that kind of continuous fresh water supply and we would have certainly lost all of those fish, but his point was very illustrative of the real needs of fish for survival and growth. Good water and good food are all that is really needed.