I'm with tony.... keep a good aquarium. Water quality may be the most key to ensure a fish lives long enough to reach a mature size. Only after a stable water quality is obtained can you help it reach the mature size. Face it, a juvenile fish in poor water quality will never (unless it's very hardy) live long enough to reach full size, no matter how hard you try. There's no such thing as Miracle-Gro for fish, as far as I know.