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Why Is My Goldfish Turning Black?

Goldfish are great fish to own as they are relatively easy to take care of and quite beautiful. These fish come in different colors. But some of them turn black. And I know it can be a little bit scary for the new goldfish owner. But if you’re reading this because your goldfish is turning black, there is no need to worry. Read on to see why and how it happens, and learn a few tips as well.

1- Due To Melanosis Disease 

goldfish turning black

Goldfish will naturally turn darker as they age, usually covering the entire body including fins and tail but sometimes just parts of the body. This is known as melanosis or black spot disease in goldfish.

Goldfish melanosis is caused by a build-up of waste products in the fish’s liver and kidneys, which causes cells to die and release chemicals into the bloodstream. These chemicals react with hemoglobin in the blood, causing it to become discolored and giving the fish a dark appearance.

Signs & Symptoms Of Melanosis

The following signs suggest that your goldfish may have melanosis:

The fish has a loss of appetite or is eating less than normal; changes in behavior; lethargy; swimming near the surface of the water; lesions on fins; red or brown streaks on the skin; growths under scales (waxy bumps); pale patches of on body; bleeding from gills or mouth.

2- Due To Cold Water

goldfish turning black 2

Goldfish are hardy fish, but they do not like cold water. When you see your goldfish turning black, it is likely because the water is too cold for them.

If you have a goldfish tank with a heater, you can use this to keep the temperature at a comfortable level for your fish. If you don’t have a heater, you can try adding ice cubes or boiling water to raise the temperature of the water in the tank.

If you have an outdoor pond, you should be careful about leaving it outside during the winter months. Goldfish need warm water and if it gets too cold outside, they may turn black before winter ends!

3- Due To Nitrite And Nitrate Levels

dirty fish tank

Goldfish are very sensitive to nitrite and nitrate levels in their aquariums. If these levels are too high, the goldfish will start to turn black. This is a sign that something is wrong with your aquarium’s water quality.

What Are Nitrites and Nitrates?

Nitrites are toxic compounds that can build up in your goldfish tank if you aren’t careful about testing for them. They’re created when ammonia breaks down into nitrite, which then breaks down into nitrate. Ammonia is produced by fish waste and uneaten food, so it’s important to keep an eye on how much waste your fish are producing and how often they eat. You can check the amount of ammonia in your tank by taking a sample of water from the tank and putting it in a test tube with a water conditioner added to it (make sure you follow the instructions on the bottle). Shake it well and wait five minutes or so before looking at the results under a microscope or on an ammonia test kit.

If there’s too much ammonia in your tank, then there may be too much waste for your filter system to handle. This can lead to poor water quality overall — including low oxygen levels — which is bad news for all fish but especially for goldfish

4- Due To Diseases

Ichthyophthirius multifiliis (Ick)

Most likely your fish has a bacterial infection called “black spot” or “fin rot.” This is caused by a small protozoan called Ichthyophthirius multifiliis (Ick), but it’s usually referred to as a black spot because that’s what it looks like.

The problem with Ick is that it doesn’t always look like spots. Sometimes the infection can affect the gill filaments and make them look white instead of red; other times it affects the fins so they look all frayed and torn up; sometimes it will cause lesions on the body or even inside the mouth and gills. In all cases, though, if you see any signs at all of these problems on your goldfish, then you should assume they have an active case of black spot and treat them immediately!

Cushing’s Disease

Cushing’s disease is an endocrine disorder that causes internal tumors to grow on the kidneys and liver. These tumors release large amounts of hormones into the bloodstream, which can cause changes in appearance as well as behavior changes such as hyperactivity and aggression.

5- Due To Stress

why is my goldfish turning black

If your goldfish is turning black, the fish may be under stress. There are several reasons why your goldfish might be stressed and turning black.

Goldfish are very resilient, but they do need to feel safe in their environment. If you have recently moved or added more fish to the tank, this could be causing your goldfish stress. The best thing you can do is take a step back and give your fish time to adjust to its new environment.

If a dead fish was left in the tank too long, or if there is an ammonia spike due to overfeeding or insufficient water changes then this could cause stress. An ammonia spike will kill off all of the beneficial bacteria in your tank which helps keep ammonia at safe levels. Without these bacteria working properly then ammonia levels will rise quickly and start poisoning your fish!

If these are not the reasons for your goldfish stress then there are other possible causes for stress that may require immediate attention.

6- Due To Overfeeding

Overfeeding is a common cause of black spots on goldfish. This disease is caused by anaerobic bacteria, which can enter the fish’s body through the mouth or gills. It is usually seen in fish that are fed too much and at one time.

7- Due To Poor Diet

Not only does overfeeding lead to black spot disease in goldfish, but also a poor diet can increase your fish’s risk for this type of infection as well as other diseases like swim bladder problems and fin rot among others which can lead to death if not treated properly.


There are a number of reasons why your goldfish may lose their color, and as you can see, none of them are overly serious. However, if the blackness spreads to other fins or to the body itself, you may need to take your fish in for a proper diagnosis. Fortunately, most causes of blackened fish only require some basic tank maintenance. Just make sure to check into the cause before you assume that it is normal for your breed of goldfish.