[PapaBear]

I wholeheartedly agree with Dicks observations.

Even if we look only to the Japanese breeders for guidance, we see breeders from all climate regions producing Koi of similar bloodlines to the same size at full maturity. Some make heavy use of heated greenhouses, others deep mudponds for overwintering. Time and a healthy environment for Koi allow them to reach their genetic potential.

There are plenty of examples, although a few are a bit extreme.
Body builders for example will reach a "plateau" of sorts, and when they do they often shift their workout routine to break through to new building of mass. More and Heavier weights aren't always the answer though. Quite often it is less weight and more repetitions. It sounds a bit like Dicks example of Koi that had stopped growing in one pond hitting a fresh spurt when placed in a new environment. It may not mean that the old environment was "bad" per-se, only that a change was needed to encourage a metabolic awakening.

Last year we observed that some of our fish merely maintained themselves during the winter fast, growing maybe 1/4"-1/2", while others added surprising length even as they slimmed down. Their individual genetics are the only possible determining factor, as they were all in the same pond, fed the same way up to the beginning of the fast, and had the same environment throughout.