Black pond fish such as koi, goldfish and carp may change colour over time. There are several reasons for this, however, colour change throughout the life of the fish is a normal process. Possible colours include black, white, red and orange. Some colour changes may be due to poor water quality or illness.
It is natural for koi and goldfish to change colour over time. Pigments in the fish's skin and scales spread out and diminish over time, which changes the colour of the fish. As the fish ages, the colours may lighten over time and eventually fade to white.
Special colour enhancing food can be fed to pond fish to change and enhance their colour. The food contains vitamins such vitamin E and C which may change the fish's colour.
Dull colour 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 build-up from improper filtering may lead to sickness and loss of colour. Water must be changed on a regular basis to keep it clean for the fish.
If a young fish's colour fades, medicated foods can help get rid of parasites or bacterial infections. Sick fish will also have mucus build-up. Parasites will cause a fish to rub on rocks, causing injury and white spots. Colour change that looks like a growth, fuzz or hair may be a bacterial infection.
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.