According to Rochester Institute of Technology's Goldfish Sanctuary, black spots on a goldfish are not well-understood. Black spots are thought to be indicative of healing scabs after ammonia burn, which occurs when water quality is low due to infrequent water changes.Continue Reading
Rochester Institute of Technology says that the best way to treat black spots or smudges is to ensure that the goldfish has clean water at all times. Goldfish owners should use testing kits to check for ammonia, nitrites and nitrates in the water and change tank water frequently to keep ammonia and nitrites to a minimum.
According to Rochester Institute of Technology, changing one-third of the water in a goldfish tank on a daily basis until the black spots have disappeared is sufficient for treatment of black spots or smudges, even without medication.
In addition to ammonia burn, black spots can occur as a result of the goldfish scraping or injuring itself on sharp objects inside of its tank, such as decorations or gravel. More rarely, black spots can occur in goldfish as a result of aquatic snails in the tank having come into contact with bird feces, according to Rochester Institute of Technology. In some cases, the black spot may remain as scarring, but in most cases it should heal with regular water changes.Learn more about Aquatic Pets