I've heard their slow as mollasses in growing. Maybe they get a certain length and slow down. Mines been 5 inches for a good 6-7 months maybe longer. I bought him when he was 5 in. One thing though, I seriously have no idea what he eats of even if he eats. He's in the 125 now in this white PVC part and he's alwas in the exact same part every morning and night.
Maybe if he was in a single tank(10 gallon) with no other fish trhen maybe he would grow bigger but I've seen no growth whatsoever.