Gold Fish Turning Black: Is This Something to Worry About?

The bright golden appearance of goldfish is one of the things that makes them so adorable. However, sometimes you might notice them turning black. Should you be worried about goldfish turning black? What does this color change mean? And what actions should you take when your goldfish turns black?

Here’s our in-depth exploration into goldfish turning black—we explain the common reasons behind it and what you need to do to keep your goldfish safe and healthy.

Common Reasons

Goldfish turning black is pretty typical for goldfish keepers. You may notice dark spots popping up on the fins or body of your goldfish. A dark vignette may also begin emerging from the edges of the fins.

It’s critical that you take the darkening of your fish seriously. In most cases, when your fish starts to darken up, it’s an indication that there’s something wrong. It’s likely the fish is suffering internally or is experiencing a health complication that needs to be addressed urgently for it to stay alive.

However, not all instances of goldfish darkening are a sign of a life-threatening health complication. The color changes could be part of a natural process. Therefore, it’s vital that you investigate the cause of the color changes before taking any action. The most prevalent reasons for goldfish darkening up are:

1. Healing from Injury

A fish’s body always tries to heal after an injury, similar to a human’s. When you get injured, scabs and scar tissue will develop at the point of injury as part of the healing process. The scar tissue in the goldfish is black. So, the black patches could be an indication that your fish was injured and is healing.

If you are aware that your fish was recently injured, then the healing tissues are the cause of the dark spots. If you aren’t aware of your fish suffering any injuries, you should look into what could have injured them. The injury may have been caused by an object in the tank, a tankmate, or in transport.

If possible, get rid of the injury risk and monitor your fish closely throughout recovery. If the wound is getting darker, which means it is healing on its own, you don’t need to do anything.

2. Ammonia in the Tank

Maintaining the proper aquarium nitrogen cycle is essential for the well-being of your fish. This process involves having sufficient beneficial bacteria in your aquarium to effectively convert toxic nitrite and ammonia into safer nitrate.

The proper nitrogen cycle will not be achieved if your aquarium has insufficient beneficial bacteria. As a result, waste made by fish and other invertebrates in the tank will not be safely converted, leading to ammonia levels rising to deadly proportions. Elevated ammonia levels cause chemical burns to the flesh. The chemical burns will manifest as black fins or black spots on your goldfish.

Other signs of ammonia poisoning in fish are:

  • Redness around the gills
  • Lack of appetite
  • Lethargy
  • Gasping for air

In most instances, these symptoms will manifest immediately when the ammonia levels in the tank become too high. The symptoms can be chronic if the ammonia levels are consistently slightly elevated.

Make frequent water changes to prevent or treat ammonia poisoning. Change about 20% of your tank’s capacity every week. Additionally, make sure your filtration system works optimally, use detoxifiers, and supplement beneficial bacteria to stabilize the water.

3. Genetics


The shimmering gold color of the goldfish is a result of millennia of selective breeding. This selective breeding is never perfect, and some goldfish are genetically inclined to change color at some point in their life. Most change color to white or a more vivid orange or yellow, while some darken up or develop dark spots. The change often occurs as the fish transition from juveniles to adults, between their first and second years.

“Mixed breed” goldfish are more likely to change color due to genetic inclinations. These are the most common goldfish on the market and are typically cheaper. When you purchase them, you may notice that some have developed unusual coloration.

Even “purebred” goldfish can change color due to genetic influences, but the changes are often more subtle. You will need to go to a reputable breeder to find “purebred” goldfish, which are typically more costly. If your goldfish is changing color due to genetics, you have nothing to worry about.

4. A Camouflage

Camouflage is among the fascinating animal superpowers. It allows an animal to better blend into its environment and be less visible to potential prey and predators. Are you aware that goldfish can camouflage?

If you place your tank in a dark environment, the fish’s pigment cells will produce melanin to help them blend in. The melanin may cover their fins, tail, or entire body. The darker the environment, the more melanin the fish will produce and the darker it will be. In some cases, the goldfish will shed all its natural colors and become white if the tank isn’t exposed to natural light. Out in the wild, this camouflage ability helps goldfish hide from predators.

Place your aquarium in a spot with some natural lighting to prevent your goldfish from losing color. You should also install aquarium lights on your tank to minimize the possibility of your goldfish turning black. These remedies will also work to restore your goldfish’s vibrant colors.

5. Disease

The two primary ailments that can cause your goldfish to develop dark spots are fin rot and black spot disease. Fin rot is a bacterial or fungal infection resulting from poor water quality and unfavorable aquarium conditions. On the other hand, black rot disease comes from a freshwater fluke parasite.

Fin rot has no specific cause; instead, it’s a secondary infection that points to a more significant problem with the goldfish, such as stress due to poor water parameters. For instance, the goldfish might develop fin rot after suffering a scratch injury in an aquarium with poor water conditions. Since the poor water parameters have already weakened the fish, the infection will take hold, whereas a healthy fish would have easily fought off the infection.

Some common symptoms of fin rot are:

  • Inflamed and open sores
  • Discoloration
  • Frayed fins
  • Lack of appetite
  • Lethargy

You could also buy fish that have already been infected with fin rot disease. Therefore, you must watch all the new fish you purchase for signs of the disease for the first few weeks. If you notice any symptoms, use antibacterial medicine to treat the ailment.

The freshwater fluke parasite (Turbellaria flatworms) that causes black spot disease spreads through water snails. Its larvae typically burrow into goldfish skin, creating black-pigmented spots as the burrowed areas heal. This ailment is less common for goldfish in indoor aquariums, mainly affecting those in outdoor ponds. This is because the flatworms also need birds to complete their parasitic cycle.

6. Overfeeding

Overfeeding your goldfish hastens the rate at which ammonia builds up in your water. Even if your tank has good beneficial bacteria, you use detoxifiers, and your filtration system works fine, they might be unable to eliminate all the excess waste from constant overfeeding. As the ammonia levels in the water increase, chemical burns will start appearing on your goldfish.

Moreover, leaving food in the aquarium for the whole day is enough to cause a problem. This is why you should only feed your goldfish once a day, and the serving should be what they can finish in a couple of minutes, never the whole day. Overfeeding can also cause your goldfish to be overweight, resulting in other health complications.

You are more likely to overfeed your fish if your aquarium is overstocked. Therefore, only keep a decent number of fish relative to the size of your aquarium.

7. Stress

Stress can also contribute to your goldfish’s darkening, primarily its scales and fins. Goldfish can become stressed due to poor water conditions, living with a hostile tankmate, strong water flow, long shipping hours, and a new environment. You can remedy the change in color by removing the negative stimuli. As soon as your fish start feeling less stressed, they will begin to regain their color.

If your goldfish doesn’t regain color even after they are less stressed, they might have contracted a bacterial infection during the ordeal. Stress weakens the immune system of fish. Treat bacterial infection with anti-bacterial medication while maintaining a stress-free environment for your fish.

Can They Regain Their Natural Color?


Whether they will regain their natural color depends on the color change’s cause and prognosis. For instance, if the color change results from genetic influences, the goldfish will stay the way it is for its lifetime. The good thing is that the color change due to genetics will not impact their health or quality of life.

On the other hand, if the darkening of your goldfish results from ammonia poisoning, you should be more concerned about helping your fish survive. Most fish don’t survive ammonia poisoning. However, you shouldn’t just assume your goldfish will perish after an episode of elevated ammonia in the tank. If the fish are increasingly active and perky during recovery, they will likely survive.

Remember that black spots indicate healing; if your fish survive, they will surely get back their vibrant color. Nevertheless, recovery might take some time. While at it, take steps to prevent future ammonia spikes.

For all the other causes of goldfish turning black, such as camouflage, stress, and disease, the prognosis is not as grim as ammonia poisoning. Taking corrective action to address the cause of the darkening will enable your goldfish to get their color back.

Final Thoughts

Now you have all the knowledge you need to prevent your goldfish from turning black and remedy any color changes that may have occurred. Keep in mind that raising fish in an aquarium is an everyday engagement. The more vigilant you are about monitoring the well-being of your fish and performing maintenance on the aquarium, the healthier your fish will be.

While goldfish changing color might be a bit worrying at first, if you are doing monitoring and aquarium maintenance right, it should be no cause for concern. Assuming your goldfish are healthy, appreciate this fascinating natural occurrence. It’s all part of goldfish-keeping fun!


Let’s address some of the frequently asked questions about goldfish.

Why is My Goldfish’s Mouth Turning Black?

The three primary causes of your goldfish’s mouth turning black are ammonia poisoning, poor water quality, and injury. Ammonia poisoning causes chemical burns all over the fish’s body in contact with water, including the mouth. The mouth turning black indicates the tissue is healing from the chemical burn.

When the water quality in the tank is poor, the impurities can also turn the mouth of your goldfish black. Your goldfish can also injure its mouth by facing off against an aggressive tankmate or nibbling objects in the tank. Its mouth will turn black as the injuries heal.

Why is My Goldfish’s Tail Turning Black?

The tail of a goldfish can turn black, mainly due to fin rot disease. Your goldfish can develop fin rot disease because of poor tank conditions and a weakened immune system resulting from stress. If left untreated, the disease will most likely spread to your fish’s other fins. Therefore, as soon as you notice the tail of your fish turning back, treat it with anti-bacterial medicine.

You should also improve your tank’s conditions by performing regular aquarium maintenance. Furthermore, remove any other negative stimuli that might be stressing your fish. This will give the fish a more robust immune system to fight future fin rot infections.

Why Do Goldfish Lose Color?

Genetics plays a huge role in the color of your goldfish. If your goldfish seems healthy and active, the color change might be a natural process influenced by genetics. In such instances, your goldfish will be okay.

Your goldfish could also be losing color to camouflage in its darker environment. Place your aquarium in a well-lit area to prevent this. Always add aquarium lighting to your tank, as it can help your goldfish have more vibrant colors. Besides, a well-lit aquarium looks much better than a dark one.

Further Reading: