The most common reasons for goldfish to turn black are genetics and ingestion of colored food. Dark environments also cause goldfish to produce additional melanin, which results in a change in pigment. Disease is uncommon, but in certain cases, diseased goldfish turn black.
Natural color change in goldfish is no cause for concern. Low-cost goldfish that are commonly found in pet stores are prone to change color, as their color genes are often mixed and their genealogy is uncertain. A common mistake that goldfish owners make is immediately assuming a black goldfish is diseased and treating it with antibiotics. While disease is a possible cause, there are several other completely natural reasons for the color change, and antibiotics can do more harm than good.
Ammonia is created by rotting food in the fish tank as well as goldfish waste. It initially causes the goldfish skin to burn, but the burns are not visible. When the ammonia level drops after the fish tank has been cleaned and sanitized, the goldfish's skin begins to heal, and the healed patches turn black.
Black-spot disease is caused by a parasite found in snails. When snails are present in the fish tank, they can pass the disease onto the fish. Symptoms of black-spot disease include black cyst formations and dark patches on the goldfish skin.