Goldfish Turning Black: 5 Causes & Treatment

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If you have a goldfish, rarely will a day go by without you taking a peek at them. Because of their playful swimming and social behavior, they always get our attention.

Glancing at our goldfish daily is not just about fun and enjoyment. Aside from being aware of the condition, looking for changes will not be difficult.

Some changes we commonly see in goldfish are weakness in swimming, damage in the fins, and in some cases, change in body color.

goldfish turning black
Source: @charliethefancygoldfish

But how about goldfish suddenly turning black? What does this mean? Is this a sign of illness? Should we be concerned?

Join us in this article as we investigate what causes our beloved goldfish to turn black.

Common Causes of Goldfish Turning Black

Ammonia Burn

Topping the list of the common causes of goldfish turning black is ammonia burn.

How is this possible?

Ammonia is a by-product of metabolism in goldfish and other fish as well. While feces and urine have a small amount of ammonia, a significant amount of this toxic chemical is released in the gills and decomposing matter like dead plants and uneaten food.

In other words, as long your goldfish is breathing and you feed them with commercial foods, ammonia will always be present as one of the chemical components of your water.

Naturally, goldfish can tolerate ammonia, but only up to a certain level. The ideal ammonia level for goldfish is between 0 – 0.25 ppm. In this range, your goldfish will have no problem.

However, if ammonia exceeds the recommended level, it will directly decrease the pH, causing your water to become acidic.

In layman’s terms, high ammonia levels cause a chemical burn, primarily damaging the skin of your goldfish. The effects are similar to placing a drop of acid on human skin. Instead of redness and scars in humans, the burned skin in goldfish will turn black.

Like scars in humans, the darkening of skin in goldfish is a sign of healing.

Ammonia burns can occur anywhere in the goldfish’s body. Although skin darkening due to ammonia burn commonly appears in the goldfish’s central body, it can also happen in the head, eyes and tails.

Considered as a minor effect, ammonia burn is just a superficial effect on high ammonia levels. However, if ammonia is left untreated, your goldfish will suffer other illnesses like lethargy, loss of appetite, and difficulty breathing.

Ammonia poisoning is the worst case scenario where the consequences are often fatal.

How to Determine?

Although ammonia burn is commonly associated with your goldfish turning black, it is not always the case. May we repeat that ammonia burns, though it is a common cause, there are other causes why your goldfish is turning black.

It is best to be sure and come up with the correct diagnosis.

Test the Ammonia Level

The easiest and cheapest way to measure the ammonia level in your tank is with ammonia test strips.

All you need to do is dip the test strip into your tank water and allow for a few seconds to react. Once a reaction has occurred, you can determine the actual ammonia level by matching it with the provided color code chart.

If the ammonia level is between 0 to 0.25 ppm, then you cannot conclude that the darkening of your goldfish is caused by ammonia burn. However, if the level exceeds the ideal concentration, then it can be a piece of conclusive evidence that ammonia has chemically burned the skin of your goldfish.

Test Water pH

A litmus paper (or any pH test strip) is the easiest and cheapest way to determine your tank’s pH level. Just dip the strip for a few seconds, allow for a reaction, and match the color with the provided chart.

If your pH reads anywhere from 6.5 to 7.5, then you have nothing to worry about as it is within the normal range for goldfish. However, if you have a lower pH, then it means that ammonia has already influenced your water to become acidic and that it is the cause of the chemical burn.

Note: Most test strips available nowadays are multi-scope. A single strip can read several parameters (like pH, ammonia, nitrite, and nitrates). We recommend you choose a multi-parameter test strip to save time, money, and effort.

If you suspect that your ammonia level is high and that there is no sign yet of chemical burns in your goldfish, then you can consider ordering online. However, if the symptoms are visible, buy the test strips at your local pet shop since ordering online may take days, and it may be too late once it arrives.

Contributing Factors: Overfeeding

If you put in more food than recommended, goldfish will eat more and release more ammonia in the water as a byproduct of metabolism.

Moreover, excessive feeding leaves behind unconsumed food. When they start to decompose, these supposed growth enhancers will become the very source of ammonia that we don’t want in a tank.

The ideal feeding frequency of goldfish is 2 to 3 times a day. As to the amount, pour in food equal to the size of your goldfish eye or what they can consume in two minutes.

Treatment for Ammonia Burn

You will do nothing to your fish. Concentrate on lowering the ammonia level and stabilizing pH to neutral.

The best treatment for ammonia burn is water change. Depending on the degree, you can do as much as 25 to 50% water change.

Consequently, you can add activated charcoal or any form of chemical filtration. Specifically, using chemical binders like ammo lock that binds with the toxic NH3 of ammonia and converts its non-toxic form (NH4 or ammonium).

Since changing the water requires time and effort, you can use drops from commercially available reagents like the ammonia detoxifier and pH stabilizer.

Physical Injury

If ammonia burn is a form of chemical injury, then your goldfish can also suffer from physical injuries.

The playful behavior of goldfish is not often a positive trait. Sometimes it causes injuries from a collision, fin-nipping, fighting, predation, and mating.

It is not necessary that only fellow goldfish can inflict physical injuries since other fish living in the same tank can also harm your goldfish. Especially if you’re careless, human handling can also hurt your goldfish.

Bruises are the immediate sign of a physical injury. In comparison, human bruises are dark red to violet, while goldfish bruises are black.

If the cause of the physical injury continues, then expect that more bruises will come out. However, if you address the problem, then you can expect that the dark bruises of a goldfish will start to heal completely.

How to Determine?

If you have checked the ammonia and pH levels and the results are within the range, yet your goldfish is turning black, there might be other reasons that cause skin darkening.

If you have a larger or more aggressive fish living with your goldfish, they may be the culprit of the injury. Spend a few minutes observing and eventually find out if one of your tank residents is attacking your goldfish.

Goldfish mating can also cause physical injury. During mating season, male goldfish aggressively pursue the female, and ramming them to release eggs is part of the process.

In case you manually handled your goldfish (during transfer or cleaning), you may ask yourself: Was I careless? Did I hurt my goldfish?

If one of these events happened before your goldfish turned black, then you can conclude that physical injury is the cause of their skin darkening.

Treatment for Physical Injury

Treating skin darkening due to physical injury is easy. The first thing to do is take out the culprit, like aggressive, larger fish and fin-nippers. However, there is an exception during the goldfish breeding season where you opt not to separate mating partners.

After the culprits are separated, you can expect bruises to start healing. However, if the wound is more than superficial, giving medicines would help. It is important, especially for injuries related to fin-nipping, as it may lead to fin rotting if not treated.

Genetics in Cross Breeding

Most varieties of goldfish we see today are the results of cross-breeding.

Since there is a mixing of genes in cross-breeding, goldfish are genetically wired to change color.

The changing of body colors in cross-bred goldfish usually happens after the first year when they shift from juvenile to adulthood.

Goldfish turning black due to genetics is different from those caused by ammonia burns and physical injuries. Aside from the gradual color change that may take months to years, you don’t need to do anything to address color change. It is perfectly natural.

Other Causes of Goldfish Turning Black

Black Spot Disease

Chemical burns and physical injuries are the leading causes of your goldfish turning black. But on rare occasions, illnesses like black spot disease can also be a reason.

Black spot disease is an infection driven by a parasitic fluke. When the parasite enters the skin, it burrows and lays eggs. Initially, the infected part will show no signs of infection. The skin color remains the same. However, when the eggs harden, it turns into a dark cysts which is the black coloration we see in the goldfish skin.

A few black spots indicate only a minor parasitic infection. However, you should be concerned if it is widespread and covers a significant part of the body since it is already an end-stage infection, and the likelihood of death is high.

How to Determine?

Like any other living organism, parasites have a life cycle. In the case of the parasite behind the black spot disease, they start by infecting birds. As birds drop their parasite-infested feces in the water, detritus feeders like snails consume them, which in turn, they also get infected.

Eventually, your goldfish will also get infected since they share the same environment with snails. Once the fish dies, birds consume them, and the cycle repeats itself.

The likelihood of a blackspot disease happens if you have an outdoor setting or having snails with your goldfish.

In the case of an indoor aquarium, the only way for blackspot disease to occur is by taking care of snails that are already infected.

Another conclusive indicator of black spot disease is constant scratching. Well, goldfish have no hands for scratching. Instead, they rub their bodies on the glass, rocks, and other solid objects. Scratching can also cause physical injury, especially if it is intense and out of control.

Treatment for Black Spot Disease

The key is to break the life cycle of the parasite. Taking out the snails will do the trick. It will benefit not just your goldfish but other fish residents as well.

Once the snails are out, you can administer antibiotics like prazipro directly to your tank. However, if it is a major infection, you might consider taking your fish into a quarantine tank and doing the medication there.

Poor Lighting

Goldfish are sensitive to light. When the surroundings are dim, your goldfish will produce melanin and become darker.

Not only this. Poor lighting will cause your goldfish to become weak and less active as dark environments signal them to calm down and sleep.

White daytime LED light is the ideal lighting system for an indoor goldfish aquarium. Aside from providing enough brightness, it also mimics the direct sunlight from its natural environment.

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Can Black Spots Develop when Goldfish Are under Stress or Sick?

Yes. More stress means more black spots. Stress in goldfish is often associated with overpopulation and variation of size.

When you have a lot of fish (not necessarily goldfish) with varying sizes, bigger fish will dominate, establish their territory and bully smaller fish.

The same thing will happen to your goldfish if they are sick. Two reasons why goldfish get sick are fin rotting and parasite attack, and both produce black spots.

Why Do Some Goldfish Turn Black and Some Don’t?

The immune system of several goldfish living in a single tank is not uniform. Some goldfish may have a healthy immune system, while some are weak.

Goldfish with weak immune systems are easily affected by stress, and black spots will show. Healthy goldfish don’t have black spots since they can withstand stress.

However, if the cause or stressor persists, then the immune system of a healthy goldfish will deteriorate, eventually producing black spots.

Will the Black Spots on My Goldfish Fade Away?

Yes, but not instantly. It takes time since the fading of the black coloration is gradual.

Remember that black spots are a sign of healing, and healing in goldfish takes time. Occasionally, it will not be restored to its original color and leave behind a mark similar to human scars.

Can Black Moor Goldfish Get Black Spots?

It doesn’t mean that black moors and other black-colored goldfish will not get black spots. Yes, they do get black spots.

The only difference is, since they are naturally colored black, the black spots are not as pronounced when compared to bright-colored goldfish.

If you focus your eyesight, you will see that the black spots in black moors are darker silhouettes of its already dark colored bodies.


Behind all the causes (common or rare), goldfish turning black is a natural response. It is a good sign since your goldfish was able to adjust, adapt and survive the stressors.

Addressing the causes is more important than addressing the darkening of the skin itself.

And by the way, if you have examined all the causes and they are not present, yet your goldfish is turning black, you can now associate it with growing old. Yes, goldfish do change color as they grow, and the darkening of their skin is part of their aging process.

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