Water quality affects the rate of growth because koi lose their appetites and may even stop eating if their environment is poor. Poor water quality can also affect the fish’s metabolism, thus hindering digestion of food.
The stocking density of the pond can also have marked effects on koi growth rate. In a lightly stocked pond, koi will become sexually mature while still of a relatively small size (25-3Ocm/10-l2in) and once mature, growth is retarded.
Although koi in a densely stocked pond will mature at a much larger size (50-60cm/20-24in), competition for food will slow the growth rate, food will be more scarce and the ‘battle’ for it can stress some koi. You will have to decide on optimum stocking levels for your own koi pond bearing this in mind. In fact, these considerations are really most important for koi farmers.