Feeding and Growth Rate
The feeding pattern, and thus the growth rate, of koi depends on many factors, such as water temperature, water quality, stocking density and genetic background Koi feed most actively at temperatures in excess of 15c(59F), thus sexually immature fish can grow rapidly during the summer months when the temperature is warmer.
Once koi are mature, their growth rate slows considerably; in sexually mature fish, most of the food eaten is utilized in producing eggs or sperm in preparation for breeding. However, unlike many other vertebrates, fish continue to grow throughout their lives and it is easy for pampered koi to reproduce and continue to grow because of their artificially high feeding rates.
The cycle of rapid growth in summer and retarded growth in winter produces rings on the fish's scales (much like the growth rings of a tree) and a koi's age can be determined by counting these rings. Some koi-keepers, however, who heat their pools in the winter, continue to feed their koi throughout the year. If a koi has continued to feed during winter its rings will not be clearly defined and it will therefore be difficult to age.