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Genetics
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It is natural for koi and goldfish to change color over time. Pigments in the fish's skin and scales spread out and diminish over time, which changes the color of the fish. As the fish ages, the colors may lighten over time and eventually fade to white.
Food
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Special color enhancing food can be fed to pond fish to change and enhance their color. The food contains vitamins such vitamin E and C which may change the fish's color.
Water Quality
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Dull color in a younger fish is not normal. The pH of the water should be appropriate for the type of fish in the pond. Levels of ammonia and nitrate buildup from improper filtering may lead to sickness and loss of color. Water must be changed on a regular basis to keep it clean for the fish.
Illness
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If a young fish's color fades, medicated foods can help get rid of parasites or bacterial infections. Sick fish will also have mucus buildup. Parasites will cause a fish to rub on rocks, causing injury and white spots. Color change that looks like a growth, fuzz or hair may be a bacterial infection.
Injury
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Fish may lose scales due to scrapes from items in the pond. They will secrete protective mucus at the injury's location. The area may lose or change pigmentation after the trauma. Several white spots may indicate scratching to remove parasites.
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Why Is My Black Pond Fish Turning Colors?
Black pond fish such as koi, goldfish and carp may change color over time. There are several reasons for this, however, color change throughout the life of the fish is a normal process. Possible colors include black, white, red and orange. Some color changes may be due to poor water quality or illness.