Goldfish Turning Black? Find Out WHY (Treatment Guide)

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Dr. Mollie Newton
Published by Dr. Mollie Newton PHD| Senior Editor
Last updated: April 11, 2024
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In this comprehensive guide, learn everything you need to know about why goldfish turn black and how to prevent it. Don’t wait until it’s too late. Understanding why your fish is changing is essential to keeping them happy and healthy.

Article Summary

  • Goldfish turning black could be a sign of a health issue, and it’s essential to determine the cause to keep them healthy.
  • The most common reason for goldfish turning black is ammonia burn, but stress, injury, environment, and disease can also lead to black patches or a completely black appearance.
  • To prevent goldfish from turning black, maintain a healthy tank environment with good filtration, avoid overcrowding, quarantine new fish, and provide adequate lighting.

Why is my goldfish turning black?

There are many reasons why your goldfish could be turning black. Ammonia burn is the most common, but other factors such as stress, injury, or environment could be the cause. For some varieties of goldfish, turning black is a natural part of their life cycle. If you notice your goldfish turning black, your first step should be to determine the cause.

What does it mean when a goldfish starts turning black?

While it is natural for some goldfish to develop black spots or turn black completely, it is a sign of a health issue for most goldfish. Ammonia burn is the most common cause of this. However, stress, injury, environment, and disease can all lead to your goldfish developing black patches or even turning completely black.

Steps such as testing your water quality, observing your goldfish’s behavior, and inspecting your fish tank can all help you determine the reason behind why your goldfish is turning black.

Is this something I should worry about?

Possibly. It depends on how quickly your goldfish is turning black. If the change occurs rapidly, this is likely a cause for concern. Your first step should be to pull out your ammonia test kit and check for ammonia buildup. Ammonia burn is the most common reason goldfish turn black in aquariums. Even a small concentration of ammonia buildup (more than 2 ppm) can harm your fish or even kill small fish.

Check your tank environment

If your ammonia levels are fine, your tank’s environment may be to blame. Are there sharp, spiky decorations that might injure your fish? Could strong water flow, incorrect water temperature, or introduction to a new tank be causing stress? Make sure to observe your fish’s behavior, too. Are they being bullied by a tank mate, flicking their body strangely, or frequently rubbing against items in your fish tank? If so, injury or disease may be the reason you see your goldfish developing black spots.

This being said, if your goldfish turns black, and develops dark pigment or dark spots slowly over a long period of time, it’s likely just a sign that it’s maturing. As goldfish age, it is normal for them to develop black marks or black coloration. For some varieties such as the Black Moor, their entire body will become black once they reach adulthood! Another possibility is that your goldfish is changing color to match its environment. This is harmless and not a cause for concern.

Goldfish turning black: causes and cures

Goldfish Swimming Together
Goldfish Swimming Together

While there are plenty of reasons why your goldfish may turn black, the good news is that the problem can be fixed in most cases. Unless your goldfish is experiencing a natural color change as part of its life cycle, there are steps you can take to stop your goldfish turning black.

Ammonia Burns

If you notice your goldfish turning black quickly, ammonia burn is likely to blame. Ammonia is a toxic chemical to fish and overexposure can lead to fatalities. Chemical burns are a symptom of ammonia poisoning. As these burns heal, black spots will form in the affected area. These can be on the tail, fins, and body of the fish. Other symptoms of ammonia poisoning are labored breathing, inflamed or reddened gills, red streaks in the fin, and loss of appetite.


Other fish in the same tank are susceptible to ammonia burns. If you notice your goldfish has symptoms you should carefully watch the others.

Ammonia Poisoning

If you suspect that your pet goldfish is experiencing ammonia poisoning, it’s important to act immediately. You must catch this issue in the early stages before it becomes fatal. Use an ammonia test kit to check your tank’s level. It takes as little as 2 ppm ammonia to harm fish and anything over 1 ppm should prompt you to take action.

Immediately conduct a 50% water change in your fish tank if ammonia levels are too high. This will dilute the amount of ammonia in the tank. It may be necessary to change the fish tank water several more times over a short period to bring levels into a safe range. You should also consider moving your goldfish to a quarantine tank. Black patches mean it has already been burnt by the ammonia and may require medicine to treat it.


If you start noticing thick mucus patches on your goldfish it might be a sign of ammonia burn. The cells produce this mucus to try to help soothe the burning skin.

Avoiding High Ammonia Levels

Causes of High Ammonia Levels
Causes of High Ammonia In Aquariums

Because ammonia poisoning is so deadly to fish, it’s important to take steps to prevent its accumulation before it becomes toxic. Several factors may cause ammonia spikes in fish tanks or goldfish bowls. Potential culprits are fish waste, dead plants or decaying plant matter, rotting food, fish poop, and dead fish.

Proper filtration is the best way to ensure your ammonia levels stay low. Without it, fish waste and other decaying matter may accumulate and cause an ammonia spike. Inappropriate aquarium size and having too many fish may cause problems as well.

Leftover food will begin to decay and raise ammonia level in your tank. A good rule of thumb is feeding your goldfish what they can eat within 3 minutes. Uneaten food should be removed from the tank. Live plants act as a biological filter and will absorb ammonia which can help reduce levels in your tank. Finally, changing your aquarium water regularly will help keep ammonia levels low. It’s recommended that you replace 25% of the water once a week.


Always treat your tap water with a conditioner before conducting water changes. Untreated tap water may contain chemicals harmful to your fish such as chlorine or ammonia.


While it is pretty rare for goldfish in an aquarium to contract Black Spot disease, it’s still a possibility. Black Spot Disease is a parasitic disease transmitted through bird droppings to aquatic snails. The snails then transmit the disease to goldfish. So while it is more likely for goldfish species in outdoor ponds or outdoor settings to contract this disease, it can still be spread through your aquarium if you have snails.

Black Spot Disease Symptoms

Black Spot disease will cause your goldfish to develop black spots on their body and fins. In particularly severe cases, nearly the entire goldfish may become black. These black spots are actually cysts that have formed around eggs laid by the parasite on your goldfish’s skin. Eventually, the cysts will burst and release more parasites into your fish tank.

To differentiate Black Spot disease from ammonia burn, pay attention to your goldfish’s behavior. When goldfish develop black spots, they will try to itch these spots by flicking their bodies or rubbing against items in the tank.

To fix the issue, remove all snails from the aquarium. This will prohibit the disease from spreading further and give your goldfish time to heal. In some serious cases, you may also consider using a product such as PraziPro to conduct an aquatic parasite treatment in your aquarium.

While uncomfortable, Black Spot Disease is not as life-threatening as ammonia burn.


When stressed, a goldfish scales and/or fins can turn black. Potential stressors include unhealthy water parameters, strong water flow, low oxygen level, incorrect water temperature, or an aggressive tank mate. If you recently purchased your goldfish, rough shipping or a long, bouncy ride home could also be the reason your goldfish is stressed. To fix the issue, remove the stressor from your fish’s environment.


Goldfish Injury
Goldfish Injury

Much like the human body forms a scab, a goldfish will form black spots when injured. This is actually a good sign! It means healing tissue is developing.

To prevent future injury, you should inspect your goldfish tank to determine the cause. Are there any sharp or rough decorations in the tank? Are other fish bullying your goldfish? These are important things to keep an eye out for.

Natural Color Changes

While your goldfish turning black can be a worrying sight, there are some natural reasons why their color can change. Both genetics of the fish species and/or a dark environment may play a role in your goldfish turning black. What’s important is to note how quickly the change has occurred. If your fish develops a black pigment over several months or years, these are likely its natural colors.


For some varieties of goldfish such as the Black Moor & Shubunkin, it is completely natural for them to develop black pigment. These varieties can transform from colorful juveniles into either completely or partially black goldfish by the time they reach adulthood.

Because this happens naturally, it is irreversible. If you want to reduce the chances of your goldfish turning black due to genetics, avoid buying varieties such as the aforementioned. Furthermore, consider buying from a higher-end, reliable fish keepers or goldfish owners.

Many cheaper goldfish species are of mixed-varieties. The coloration of these mixed breed goldfish can develop unpredictably. If you’re interested in purchasing a more traditional orange goldfish, consider a variety such as the Oranda.

Dark Environment

A goldfish’s colors will naturally change to adapt to its environment, almost like camouflage. If your tank is in a dark environment or has a dark background, there’s a chance your goldfish is turning black to match its surroundings. To correct this, move the tank into natural light or add lights to your aquarium. However, don’t expect this to immediately fix the problem. It may take several weeks for your goldfish’s scales to brighten back up.

How to Prevent Goldfish From Turning Black

Goldfish with Black Spots
Goldfish with Black Spots

Maintaining a safe, healthy tank environment is the best way to prevent your goldfish from turning black. An excellent filtration system is key to this. Having proper filtration in your goldfish tank will also help promote growth of beneficial bacteria that help lower ammonia. Poor filtration will cause fish excrement to accumulate and ammonia to spike. This is the most common reason that goldfish become black. A filtration system that offers chemical, mechanical, and biological filtration is recommended.

Adding aquarium plants and avoiding too many fish in the same tank can also reduce the likelihood of an ammonia spike. Changing the water in your tank regularly is recommended. Not permitting uneaten food to sit in the tank will also help.

Quarantining new fish and water snails before introducing them to your tank is another important step you can take to prevent your goldfish turning black. Quarantine should last for two weeks. This reduces the chance of diseases from infected water snails being transmitted to your goldfish. Goldfish can get black spot disease from snails with parasitic fluke disease.

You should also pay attention to how your goldfish gets along with its tank companions. Aggressive play or bullying may injure your fish and cause black patches to form. For the same reason, you should avoid putting items in your tank that are sharp or have a rough texture.

Finally, you should make sure that your goldfish has a bright, well-lit environment. This will prevent the color of your fish from dulling to match its surrounding. Bright, colorful decorations may also help.


Why are my goldfish fins turning black?

Goldfish fins may naturally develop black marks as they mature. If the change occurs rapidly, however, it is likely a health concern. Ammonia burns can form on both a goldfish’s body and fins. Stress or other injury could be to blame as well.

Disease could also be the root of the issue. Black spot disease may cause black spots to form on the fins of your fish. Another sign of disease is fin rot. Fin rot can cause the edges of a goldfish’s fin to turn white, brown, or black. Other signs of fin rot are frayed edges to the fin/tail, inflammation at the fin’s base, or parts of the fin/tail rotting away. Fin rot is a symptom of a disease, not a disease itself.

Why is my goldfish’s tail turning black

A goldfish’s tail turning black can occur for the same reasons their fins may turn black. While slow, gradual changes are usually due to the fish maturing, rapid changes can be a sign of ammonia burn, stress, injury, black spot disease, or fin rot.

Why is my goldfish’s mouth turning black

While this could be your goldfish’s color naturally changing as it matures, it may be a sign that something is terribly wrong. A goldfish’s mouth turning black can indicate ammonia poisoning or bacterial infection. You should check your fish for other symptoms such as fin rot, not eating, and labored breathing.

Do goldfish turn black when sad

While there is no research to indicate a goldfish will turn black when it is sad, they may while stressed. If you suspect your goldfish is turning black due to stress, closely inspect its environment for potential stressors such as strong currents, aggressive tank mates, or poor water quality.

Is it normal for goldfish to change color?

Yes! Goldfish live 10-15 years and as they mature they often change color and markings. However, it is more common for a black goldfish to develop lighter coloring as an adult fish. On the other hand, it is unusual for a yellow or orange goldfish to turn completely black with age. These changes usually occur within the first 1-2 years of a goldfish’s life. Rapid color changes in goldfish are not normal and usually indicate poor health.

Will black spots on goldfish go away?

It depends on the cause. If it’s genetic, then no. However, if disease, stress, or ammonia burn are to blame, then addressing the root of the issue may cause the black spots to disappear over time. However, this is not guaranteed, and it is possible your goldfish will remain black forever.

Why do goldfish lose color?

Goldfish may lose their color for several reasons. One, it could be genetic. Some goldfish’s color will dull as they age. Keeping your fish healthy can help maintain their color and growth. You can try feeding them color-enhancing goldfish food to improve their color.

Two, it could be due to a dim environment. Dark wallpaper, poor lighting, or a black background could cause your goldfish’s color to dull. Introduce light and bright decorations into your aquarium to counteract this.

Three, it could be a sign of sickness. Bacterial infections, Black Spot Disease, and other illnesses can all cause your goldfish to lose color. If this is the case, your infected fish will likely display additional symptoms. Four, it could be a sign that your goldfish is healing. Much like the human body becomes discolored when bruised, a goldfish’s scales may lose color while recovering from an injury.

How to increase the color of goldfish?

Introduce bright light and colorful decorations into your fish tank. A goldfish’s colors will increase to match a bright, colorful environment and, conversely, dull to match a dim one. Your goldfish’s diet can also play a role in a goldfish’s color.

Consider using a color-enhancing fish food to make them more colorful. Finally, changing the water in your aquarium regularly is necessary to ensure your goldfish is as colorful as possible.


In conclusion, the most important thing to do when your goldfish turns black is to determine the cause. Offtimes, black spots, or a full-body color change are signs of a greater issue at hand. Taking the necessary steps is usually quite simple and will improve the lives of your fish. Plus, it will make fish keeping a much more enjoyable experience for you.

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