IMO, the best way to get fish to grow quickly is through a good diet.
Give'em plenty of frozen food and other high protein stuff and they will grow. Live Tubifex worms rock but you need to be careful as they can sour a tank fast if left in the water too long. Pick up a Tubifex worm feeder and leave them in the tank for 15-20 minutes at a time. Rinse the portion and store in the lettuce crisper (Refridge) in a very shallow pan with wet papertowels. Rinse before putting back in the tank. Always use cold water when rinsing. Of course, frozen worms work too.