Koi are highly valued for their color and certain additives can be included in the diet to maintain and enhance the fish's natural coloration. Carotene affects the red pigmentation, but if used at too high a concentration, even the white pigment on the Koi will turn pink.
Spirulina platensis also enhances and fixes the red pigment, but does not turn the white to pink to the same extent. It is a type of algae found and cultured in Mexico and eaten by the people, since it contains a high level of protein. Initially, it was fed to Koi on account of its nutritional value and not because of its color-enhancing characteristics. Now that these have been established, it is fed to Koi for only one month each year, usually during September, but can be given at any time, even during colder periods, at temperatures of 10c, but no lower. Some Koi farmers feed it for the month before the fish go to market to bring out the best colors in the Koi.
Good coloration is not only enhanced by good feeding, however. Healthy fish tend to have much brighter colors than diseased ones. To bring out the color in Koi, particularly the white, you must provide good living conditions. Strong red and yellow pigments develop well in waters rich in green phytoplankton (single-celled plants). Because Koi are difficult to see in green water, it helps to feed a diet that will enhance the red pigmentation. Black pigment is enriched in hard water with a pH level of 7.5-8.5. Remember, however, that changing the pH and hardness of the water can affect the toxicity of ammonia and nitrite.