Fish can turn black as a response to discus disease, a seasonal, flu-like illness that causes the fish's skin to come off in patches and turn dark. The fish may initially have small white patches over the body, and the disease is most common after placing new fish in the aquarium. Treatment for discus disease includes treating the tank with potassium permanganate, changing the water in the aquarium daily and removing all decorations in the tank until the fish have recovered. Antihistamines that are appropriate for the fish may also be used to reduce the effects of the disease.
Some fish turn black when exposed to stressful situations, such as reduced oxygen levels, water pollution or overfeeding. To determine the cause of the stress, test the pH levels in the aquarium and make the necessary adjustments to improve the quality of the water. If the pH is normal, test the water temperature, and check the aeration and filtration systems to ensure the water is properly filtered and oxygenated. If the cause of the illness is unclear, remove and replace 50 percent of the water in the tank with fresh, non-chlorinated water. Contact a veterinarian for assistance if the condition does not improve.Learn more about Fish