Goldfish are a common pet for children and an ideal starter fish for those interested in having a home aquarium. They come in a variety of colors, including white, silver, orange and black. Goldfish can be all one color or have random patches all over their bodies, including their fins. These colors may change over the lifetime of the fish.
Developmental Color Change
Goldfish have several different types of skin cell that determine the coloration of the fish. Those present in black coloration are called melanophores and they contain melanin, a black pigment. A goldfish placed in a dark environment, such as a tank in front of a dark background with lots of dark rocks, will produce more melanin in its melanophores, thus developing black fins or other black patches. This change is a natural reaction and is not detrimental to the fish.
Black patches on a goldfish's fins or elsewhere on its body can be a sign of excessive levels of ammonia in the water. Fish waste and uneaten food are typical sources of ammonia, and if levels become too high, this burns the fish's skin. The effect is a simple change in color, rather than any visible damage. The skin turns dark brown or black as it begins to heal, so black patches may not appear if ammonia levels are constantly high because the skin does not have the opportunity to begin to recover.
Black Spot Disease
Black spot disease is rare in home aquariums. It more commonly appears in ponds. This is because fish catch the disease from bird droppings. Infected birds' droppings contain the eggs of a parasite that burrows into the fish's skin and causes discoloration. Healthy, adult fish are not badly affected. Rarely, water snails can introduce black spot disease into indoor aquariums. Treatment consists of removing the snails from the aquarium, which breaks the parasite's life cycle.
Buy the largest tank you can afford. Although many owners keep goldfish in fishbowls, these are not the ideal home. A large surface area provides more oxygen to the fish. The water temperature should be stable, so do not situation your tank in direct sunlight. Replace approximately 1/4 to 1/3 of the tank water every 3 or 4 weeks. Allow the fresh water to stand for 3 or 4 days so that the chlorine can evaporate and reach the same temperature as the tank water.