The little goldfish has been swimming happily in his bowl for months now with no issue. But now you notice his scales and fins beginning to turn black. Why is this happening, and will he be ok?

The most common reasons for a goldfish to turn black are; that your fish is healing from ammonia burns, it is adapting to its environment, or that it has black spot disease. The first two reasons are the most common and can be prevented with regular tank maintenance, and a light-colored and bright environment surrounding the tank. 

Ammonia Burns

If ammonia burns are the cause of your fish turning black, it means that your fish has already survived a spike in ammonia levels in the tank and is healing, or that the ammonia level is high, and your fish is trying to heal despite it. 

In either case, test your tank water for ammonia. If ammonia is present, a thirty to fifty percent water change needs to happen immediately to reduce ammonia levels. Follow this up with a check of the filtration system and a cleaning of the gravel at the bottom of the tank. Fish under stress will produce even more ammonia than normal, causing levels to rise even more rapidly as the tank conditions deteriorate.

If your fish turns black after a water change and cleaning, then test your tank for ammonia. If your level is zero, then you can rest assured that the color change is a result of the healing process. The black spots will not go away as the fish heals though. They are scars that will remain.

Burn Prevention

To prevent ammonia burns on your goldfish, test the tank often. Remove decaying matter and waste quickly via filtration and regular cleanings. Goldfish are not known for their cleanliness and can pollute a tank in short order if overfed. Monitor your fish and its eating habits. Erratic or lethargic swimming is also a sign of ammonia poisoning.


Goldfish are genetically designed to blend in with their environment, especially if they are less than one year old. If you have a black or dark background behind the tank, your fish will begin to change color to blend in with it. This change may manifest as a full-color change or possibly just spots and/or blotches. 

The black color comes from melanin, a pigment produced by the skin. A dark backdrop or placing lots of dark objects like rocks in the tank can trigger the fish to produce more melanin to make itself less conspicuous.

If you see this happening, test your water first to rule out ammonia poisoning, then enjoy your unique goldfish. This adaptation is something the fish is genetically wired to do. The change will be permanent, so you do not want a black goldfish, replace the dark background with a lighter one or move the tank to a brighter and more well-lit location. 

Black Spot Disease

A rare cause of goldfish turning black is black spot disease. This condition is a result of parasite fluke eggs burrowing into the skin of your goldfish. These lesions crust over with a black cyst that the fluke will burst through on its way out. 

Levels of infections are usually only a few spots, but cases have occurred where what looked like a black goldfish was really an orange one covered with these cysts. Black spot disease is passed from water snails kept in the same habit as the goldfish and is most often transmitted via birds eating infected snails and passing the flukes or their eggs in droppings that land in outdoor goldfish ponds.


A black goldfish can be a beautiful adaptation of a species to its environment or a cause for grave concern and immediate action to save the fish’s life. Test your tank often for ammonia and clean it regularly. Be mindful of the background and surroundings of your tank. And do not let your goldfish have a snail for a roommate. Take care of your fish, and he will lend beauty to your home for a long time.

Brandon Tanis