How do koi get so big in Niigata, Japan? This video profile of the koi growing area in Japan offers some interesting information on why young koi grow so fast in these natural clay ponds. One reason is the large amount of snowfall in this area:
During the months of March and April, the meters of snow that have fallen into the empty mud ponds melt. During the spring, the mud ponds fill with fresh and ice-cold melt water. When the sun warms the water, the growers can start preparing for the first Koi in the mud ponds.
First the smallest Koi and after that the larger ones. There can sometimes be weeks or even months in between. The reason? Smaller fish can do more against the temperature fluctuations during the first spring months than the bigger Koi.