I see a lot of fish hit the foot mark and slow down, not every, but most. Do you want your grouper to grow more? Don't those fish get 3 feet? That would take a monster sized tank to house it! I'd be happy he is growing slow. You could feed every other day and change up the diet more, also add vitamins etc, just be sure your bioload can handle the extra waste! I'd personally just vary his diet more and add vitamins if you aren't already doing so. Do you have any pics of the grouper? Adults are gorgeous!