The Fish Guru

Why Do Goldfish Turn Black? Common Causes and Prevention Tips

Goldfish are a beloved pet and are often the first fish that many people get. Their vibrant colors and easy care make them a popular choice for aquarium enthusiasts.

However, sometimes goldfish can unexpectedly turn black, leaving their owners confused about what went wrong. In this article, we will discuss the common causes of why goldfish turn black and how to prevent and treat it.

Common Causes of Goldfish Turning Black

Ammonia Burn: One of the most common causes of goldfish turning black is ammonia burn. This happens when the ammonia levels in the tank are too high and the fish’s skin becomes irritated, leading to a chemical burn.

Contributing factors include overfeeding, which causes unconsumed food to rot and produces more ammonia, exacerbating the issue. To prevent this, ensure that you do not overfeed your goldfish, and do regular water changes to keep the ammonia levels in check.

You can also use activated charcoal and chemical filtration to help absorb and remove excess ammonia from the water. Physical Injury: Goldfish can also turn black due to physical injury.

This can happen when a fish attacks another, causing bruises. Separating the culprits and treating the injured can help prevent this from happening.

Genetics in Cross Breeding: Some cross-breed goldfish can change color when they mature due to genetics. Their black coloration may simply be a result of their coloring genes.

How to Determine the Cause

Testing Ammonia Levels and Water pH: Ammonia levels in the water can be tested using ammonia test strips, and maintaining the ideal ammonia level is crucial. Water pH can be tested using litmus paper or a pH test strip.

If the water is too acidic, this can also cause goldfish to turn black. Ensuring the water pH is ideal will help to prevent ammonia burns.

Contributing Factors: Overfeeding: Overfeeding is a significant contributing factor to ammonia burns and can lead to the development of black spots on the skin. Ensure you feed your goldfish the correct amount and frequency, and remove any unconsumed food.

Physical Injury: While observing culprits and goldfish mating can help in determining if it is a physical injury, suitable treatments such as separation and medication should be used to cure any ailments.

Black Spot Disease: Goldfish can also develop black spots as a result of a parasitic fluke. Snails are also often a carrier for these parasites, leading to goldfish scratching themselves on tank walls and decorations, causing black spots on their skin.

Removing snails and administering antibiotics can help treat this disease.

Poor Lighting: Poor lighting can cause goldfish to turn black due to melanin overproduction. Weak and less-active goldfish may also be indicative of poor lighting.


In conclusion, understanding the common causes of why goldfish turn black can help prevent and treat the issue effectively. By testing ammonia levels and water pH, limiting overfeeding, treating physical injuries and diseases promptly while providing adequate lighting, you can ensure the continued health and vibrancy of your goldfish.

Goldfish are often adored for their vibrant colors, which can make the experience of having them as pets all the more enjoyable. However, sometimes goldfish can turn black, causing concern and confusion among their owners.

In addition to the common causes of goldfish turning black that we previously discussed, there are a few other reasons why this may occur. In this article, we will cover black spot disease, poor lighting, and some frequently asked questions about goldfish turning black.

Black Spot Disease

Black spot disease is a parasitic infection that can cause goldfish to turn black. The infection is caused by a type of fluke that burrows into the skin of the goldfish and forms a cyst.

Over time, the cyst causes a black spot to appear on the goldfish’s skin. This disease can be contracted by contact with bird feces in outdoor ponds but is relatively uncommon in indoor aquariums.

In the beginning stages, black spot disease is easy to treat. Simply remove all of the snails from the aquarium as they are often carriers of the parasites, and medicate the aquarium with antiparasitic medication.

If the infection is in its advanced stages, however, it may be too late to save the goldfish, and the black coloration of its skin may become permanent.

Poor Lighting

Poor lighting can be another cause of goldfish turning black. This can occur when fish are exposed to harsh, direct sunlight or too little light in their aquarium.

If the aquarium’s lighting is too intense or the sunlight enters the tank directly, the goldfish may produce an excess of melanin, which can cause them to turn black. Similarly, if the aquarium is underlit or the lighting is not adequate, the goldfish may become weak and less active, causing them to turn black.

To prevent this, use a white daytime LED light that is not too intense, and be sure to provide adequate lighting for the fish in the aquarium.


Here are some frequently asked questions about goldfish turning black:

Q: Can stress cause goldfish to turn black? A: Yes, stress can cause goldfish to turn black.

When goldfish are stressed, their body releases hormones that can cause the fish’s coloring to fade, and black spots can appear on their skin. Q: Is it normal for goldfish to fade in color over time?

A: Yes, it is normal for goldfish to fade in color over time. Aging and changes in the environment can cause goldfish to lose their vibrancy over time.

However, if goldfish rapidly lose their color, it may indicate a health issue that needs prompt attention. Q: Are black moors prone to turning black?

A: Based on their genetics, black moors are already dark in color. However, they can still develop black spots over time due to stress or poor water conditions.

Keeping their aquarium clean and stress-free can help avoid this issue. In conclusion, while goldfish turning black can be concerning for their owners, there are multiple causes for this occurrence.

Black spot disease and poor lighting can cause this phenomenon. However, understanding the issue and following the necessary steps to address it can help ensure the continued health and vibrancy of your goldfish.

Goldfish turning black may be initially concerning for their owners. However, several factors can contribute to this issue.

Ammonia burn, physical injury, black spot disease, poor lighting, and genetics in crossbreeding can all cause goldfish to turn black. To prevent and treat this issue, testing for pH levels, removing unconsumed food, and treating ailments promptly will ensure the continued health of your goldfish.

Proper care and understanding of the causes can ensure the continued vibrancy and longevity of your pet.

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