Turning black goldfish

Why Is My Goldfish Turning Black? Causes & Solutions

Goldfish are a popular and beloved addition to any fish tank; however, they sometimes develop unusual colorations that can be concerning. One of the most worrisome changes is when a goldfish’s scales, fins, and tail turn black. Several factors, such as disease, environmental stress, and genetics, can cause discoloration. 

Below are common causes of black discoloration and possible solutions to help your goldfish regain its natural coloration.

Common Causes of Goldfish Discoloration

Various factors cause black discoloration in goldfish and understanding these factors can lead to the right solution and better health for your goldfish. Some of the factors include:


Certain goldfish breeds, such as black moors and black orandas, have a genetic predisposition to develop black coloration on their scales, fins, and tail. It means that even if you breed the fish in optimal conditions, it’ll still have black coloring due to its genetic makeup.

When your goldfish develops black spots or patches due to genetics or a natural color change, the coloration happens gradually and doesn’t change your goldfish’s behavior. This is perfectly normal.

What isn’t normal though, is when the color darkening happens suddenly, accompanied by red marks, physical problems, or behavioral differences in your goldfish. This is considered unnatural and should be a cause for concern.


Fish can change their color to blend in with their surroundings to survive and avoid predators. For example, when goldfish are in a dark environment, such as in a tank with limited light, they may develop black coloration on their scales as camouflage. It’s natural and not a cause for concern. 

Ammonia Burns

Ammonia is a toxic chemical produced as a byproduct of fish waste, uneaten food, and other organic matter in an aquarium. Ammonia burns are a type of injury that occur in fish due to exposure to high levels of ammonia in the water.

Symptoms of ammonia burns in fish include red, irritated, or ulcerated skin, fins, and gills. The fish may also exhibit abnormal swimming behavior, appear lethargic or lose appetite. In severe cases, ammonia burns can be fatal.

Ammonia burns happen due to the following factors: overstocking the tank, overfeeding the fish, and not performing regular water changes. In addition, a lack of proper filtration or an incorrect pH level can also contribute to high ammonia levels in the water.

If you suspect your fish has ammonia burns, it’s essential to do a water change immediately and seek a veterinarian’s advice to treat the fish.


Your goldfish might darken because of stress. The stress can be due to poor water quality, overcrowding, lack of hiding places, aggressive tank mates, or environmental changes. As a result, stressed fish may appear darker, paler, or lose color. They can also develop vertical lines or bars on their body called “stress stripes.”

Stress can also weaken a fish’s immune system, making them more susceptible to disease and infection. For example, a bacterial infection can cause your goldfish’s scale to turn black.


Overfeeding can cause your goldfish to turn black. When your fish consumes more food than is needed, it can cause excess waste in the water. The resulting poor water quality causes stress and can lead to skin darkening.

Overfeeding can also cause an excess of fat in the fish, leading to a buildup of fat deposits under the skin. That can cause the skin to appear darker, especially around the belly area. Symptoms of overfeeding in goldfish include a distended belly, a lack of appetite, and a buildup of fat deposits under the skin.

It’s essential to provide the fish with food strictly appropriate for their size and activity level to prevent overfeeding-induced darkening of goldfish skin. You should also remove any uneaten food from the tank and perform regular water changes to maintain acceptable water quality.

Light Exposure

Fish can change their color in response to different lighting conditions. 

Prolonged exposure to bright light can cause stress in fish which lead to the production of a hormone called corticotropin-releasing factor (CRF). This causes melanin to be produced at a higher rate, resulting in the skin darkening.

Black Spot Disease

Black spot disease is the appearance of black or brown spots on your goldfish’s skin and fins caused by various circumstances. A common cause is allowing water snails to cohabit with your goldfish. Parasites from plant matter, debris, and other organisms’ eggs and larvae can result in black spot disease.

Luckily, the disease rarely goes unnoticed for long. Some symptoms of black spot disease might include the following;

  • Dark spots on the skin and fins
  • Scratching and rubbing against objects in the tank
  • Loss of appetite
  • Gasping for air at the surface of the water
  • Discoloration or inflammation of the gills
  • Lethargy

Also, note that poor water quality, stress and poor nutrition can exacerbate the symptoms of blackspot disease. It’s more common in crowded or dirty conditions and tanks with a high fish-to-water ratio.

Causes of goldfish discoloration


Treatments for your goldfish turning black will depend on the underlying cause of the discoloration. But there are some general steps that can improve your goldfish’s health and well-being, regardless of the cause.

Maintain Water Quality

Perform regular water changes: This helps to remove waste and other contaminants from the water, which can help to reduce stress and improve the overall health of the fish. A general guideline is to change 25-50% of the water every 1-2 weeks.

Use a high-quality filter: A filter helps to remove debris and other contaminants from the water, and it also helps to maintain good water flow, which is essential for the health of the fish.

Monitor pH and ammonia levels: The pH level and ammonia level of the water should be within the appropriate range for goldfish. The ideal pH range is between 7 and 8, and the ammonia level should be 0.

Keep the tank clean: Regularly clean the tank and any decorations, and remove uneaten food and debris, as it can cause an increase in ammonia levels.

Provide Adequate Lighting

Different species of goldfish have different lighting needs. For example, some species require subdued lighting, while others require brighter lighting. Knowing the natural habitat of your goldfish can help you provide the appropriate lighting for your fish.

When positioning the light, place it above the tank but not too close to the water’s surface to avoid overheating the fish. Ensure you provide your fish with a consistent day-light schedule, observe them to see how they react, and adjust accordingly.

Avoid Aggressive Tank Mates

Aggressive tank mates can cause stress in goldfish, leading to darkening skin and fins.

When choosing tank mates for goldfish, research the compatibility of different species. Goldfish are generally peaceful and can stay with other gentle fish such as guppies, platies, and tetras. They shouldn’t be kept with aggressive fish such as cichlids or bettas, as these fish may attack or bully the goldfish, causing stress and darkening skin.

Quarantine New Fish

Quarantining means isolating new fish before introducing them to the main tank. It ensures that the new fish are healthy and don’t have any diseases or parasites that could infect the other fish in the tank.

Quarantining new fish allows you to observe the new fish for any signs of illness or abnormal behavior and test the water in the quarantine tank for pH, ammonia, and nitrite levels before introducing them to the main tank. The quarantine period should be at least two weeks. 

Avoid Overfeeding

Here are some steps you can take to avoid overfeeding your goldfish:

Feed the appropriate amount of food: Feed your goldfish the appropriate amount of food for their size and activity level. If your aquarium is larger than 10 gallons, your goldfish will have tons of real estate to explore, so they’ll likely be more active than fish in a smaller aquarium

Provide a varied diet: Goldfish require a varied diet that includes plant- and animal-based foods. Please provide them options like flakes, pellets, frozen or live food.

Avoid feeding at night: Fish are less active at night, so they do not need as much food. 

Remove uneaten food: Remove any uneaten food from the tank to prevent it from decomposing and causing an increase in ammonia levels.

Add Live Plants

Adding live plants to a goldfish tank effectively prevents the darkening of the skin in goldfish. That’s because live plants can help to improve a tank’s water quality, provide natural hiding places for fish and reduce stress. Adding live plants to a goldfish tank includes researching suitable options, preparing the substrate, planting the plants in the tank, providing appropriate lighting, and monitoring and caring for the plants. Some suitable plants to pair with your goldfish are;

Get a Bigger Tank

In a smaller tank, the water quality deteriorates faster because waste and uneaten food accumulates quickly. The fish also may not have enough space to swim, which can cause stress and lead to the darkening of the skin. A bigger tank allows for more water volume (which means it gets dirty slower), more space for the fish to swim, and more room for beneficial bacteria and plants that help to keep the water clean.

When getting a bigger tank, consider the number and size of the fish and the type of filtration and lighting you’ll need to maintain the tank properly. Be sure not to overstock the tank, regularly perform water changes, and monitor the pH and ammonia levels.

Will Black Spots on a Goldfish Go Away?

Genetic spots will not go away, but if certain conditions cause them, they can be treated and will go away over time.

Assess Your Goldfish, Then Treat Them Accordingly

Understanding the causes of black discoloration in goldfish is essential in preventing and treating this condition. By taking a multi-faceted approach and addressing each possible cause, you can help to avoid the darkening of your goldfish’s skin and promote their overall health and well-being.

Did you enjoy this article? We hope this article has helped you understand the causes of black discoloration in goldfish and provided you with valuable tips on preventing and treating it. Please let us know your thoughts in the comments, and feel free to share the article with others.

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