Many fish do slow down their growth in a tank, because they release growth inhibiting hormones. Of course, frequent water changes can minimize this problem. In my own experience, some fish seem to grow very well in my tanks, others do not. I find that clown fish grow steadily, and eventually reach a normal, mature size. Maybe not maximum size, but normal. My ocellaris clowns are between 1.5 and 3" long, with the large female probably inhibiting the smaller males from growing much more. My previous maroon clown got huge. Since his death after about 9 years or so, its been replaced by a tiny gold stripe. I want to enjoy its infancy before it grows into a mean old bastard. On the other hand, I have found that juvenile tangs (yellows and hippos are the only ones I have kept) grow very rapidly until they double their size, maybe triple if they were real small. Then they simply stop growing. The yellows live forever...hippos only about 2 years, so I've given up on them.
