There is no way to make a fish grow faster than it was designed to grow. It's impossible. All you can do is give it optimal conditions. Young fish release a hormone that stunts growth so that they will have less competition from their siblings. Doing frequent, large water changes removes this hormone and allows the fish to grow uninhibited. A good varied diet is just as important. The person I am buying bettas from does nearly 100% water changes on his fry twice a day. From the time they are jarred, he can get them to adult size in about 2 weeks with frequent water changes and feeding high quality, nutritious foods.