Why Is My Goldfish Turning Black – 7 Reasons Why (And How To Prevent It)

Goldfish are meant to be gold. They should never start turning black or developing black patches across their body or fins. A goldfish that is turning black is not a good sign. So why is my goldfish turning black? Either there is a problem with water quality or the fish is being affected by disease or illness.

Keep in mind that some goldfish breeds, like the black moor, are meant to be black and this display of color is perfectly natural! However, developing black spots is not a natural part of goldfish aging and should be treated immediately.

Key Takeaways

  • A goldfish turning black is usually a sign of poor health or stress
  • Ammonia and Nitrite levels are the best things to check on if your fish turns black
  • Healthly goldfish should display great colors, be active, and always are hungry
  • Ensure you feed your goldfish a proper diet to keep it from getting stress or sick

A Healthy Goldfish

Before we dive into why your goldfish might be turning black, we need to first establish a baseline of normalcy.

1) Color. Goldfish come in many colors, shapes, and sizes. Their scales should always be bright and shiny, no matter which breed of goldfish you have. Dullness, cloudiness, or apparent abrasions, are not signs of a healthy goldfish and indicate an underlying problem.

2) Behavior. A happy goldfish will actively swim across the aquarium searching for food. Sick goldfish will often lay on the bottom of the substrate, and might even show signs of difficulty breathing and staying in an upright position.

3) Appetite. Eagerness is a great indicator of a happy and healthy goldfish! These fish should always be hungry and searching for food. If your fish becomes less interested during feeding times or stops eating altogether, then something is wrong.

Goldfish are considered easy fish to keep, but a lot can surprisingly go wrong.

Why Is My Goldfish Turning Black?

Your goldfish is turning black due to either water quality issues or illness, or both. In almost all cases of a goldfish turning black, high ammonia levels and subsequent ammonia poisoning are the roots of the cause.

There are a few other reasons why your goldfish might be turning black, including nitrite poisoning, fin rot, and black spot disease.

Do Goldfish Turn Black When They’re Sick?

Yes, goldfish can turn black when they are sick. The main illnesses that cause a goldfish to turn black are fin rot and black spot disease. These black patches may be due to chemical burns, parasites, or other symptoms of the illness.

Do Goldfish Turn Black When They’re Stressed?

Yes, goldfish also turn black when they’re stressed. This stress may be due to an illness that has not been listed or may be due to poor water quality. Stress can be caused by illness, incorrect water parameters, incompatible tank mates, or having too many fish in the aquarium.

Ammonia Poisoning

Ammonia poisoning is the result of excessively high levels of toxic ammonia being present in the aquarium, leading to burns on the fish’s external and internal organs. Goldfish are very messy fish and many hobbyists struggle with keeping ammonia down in the aquarium; this is the main reason why these fish are not recommended as a beginner species.

Ammonia should always read as 0 ppm in the cycled aquarium. 5 ppm is about the point where beneficial bacteria become unable to process ammonia entirely.

Many beginner hobbyists fail to cycle their aquarium fully or overstock their new tank too quickly, overwhelming the bacteria that are responsible for converting ammonia into less toxic compounds. However, high ammonia levels can also come from eventual overstocking, overfeeding, and poor tank maintenance.

Ammonia is a toxic chemical that burns through the skin and tissue of fish and invertebrates. It can quickly affect internal and external organs, leading to red and inflamed gills, body discoloration, lack of appetite, and lethargy. One of these symptoms could present as dark spots around the gills or on the rest of the body.

Eventually, organs malfunction to the point of a painful death.

Luckily, ammonia poisoning is easy to prevent as long as new aquariums are allowed to fully complete the nitrogen cycle1. Mature tanks should have an adequate filtration system for a goldfish bioload and fish tank maintenance should be regular.

Nitrite Poisoning

Nitrite poisoning is similar to ammonia poisoning and often follows if water quality is not corrected. Nitrite poisoning is when too much ammonia is converted into toxic nitrite. Nitrite (NO2) binds to hemoglobin, which causes the fish to suffocate.

Lethargy, difficulty breathing, loss of appetite, and possibly black discoloration could all be a result of nitrite poisoning. The best ways to prevent this from happening are by allowing the aquarium to fully cycle, preventing overstocking the fish tank, and performing regular maintenance to stabilize water parameters.

Fin Rot

Fin rot can be caused by bacteria or a fungus, though bacterial fin rot is much more common to see. Though this is considered an illness, fin rot is actually the result of poor water quality.

Fin rot is most likely to infect fish when the fish is already injured or stressed. The most obvious signs are inflamed and frayed fins. At any point of fin rot, a black vignette may start to form around the edges of the tail, which could make it seem like your fish is changing colors. Unfortunately, this is the rot setting in.

Lethargy, loss of appetite, clamped fins, abrasions, and discoloration often accompany fin rot. A fungal fin rot infection will have a white, fuzzy appearance instead.

Black Spot Disease

Black spot disease is the least probable cause of a goldfish turning black, but this parasitic fluke disease cannot be ruled out entirely.

Black spot disease is caused by a freshwater fluke parasite from the Neascus genus. Adults enter the aquarium or pond through infected water snails. The parasite’s life cycle then needs to be facilitated by bird droppings, so this disease is nearly impossible to find in the aquarium setting but much more likely to happen in outdoor ponds.

Once inside the system, the larvae of these parasites burrow into the fish’s skin where the skin then compensates by covering the larvae with a black spot; every speck of black on the body of the fish is a fluke larva.

In large numbers, black spot disease can easily make it look like your goldfish might be turning black. However, the more apparent the spots, the worse the case of black spot disease. Other symptoms include lethargy, loss of appetite, flashing, and scratching. It is possible for secondary infections to happen due to injury.

For the most part, black spot disease will clear up on its own due to the nature of the parasite’s life cycle. Aquarium medicines and treatments may be used to facilitate healing but are not usually necessary.

Will The Black On Your Goldfish Go Away?

Once the fish’s health has improved, the black on your goldfish will go away. Goldfish are never supposed to change color and don’t have the ability to on a genetic level, such as koi betta fish that have mutating genes.

It may take a while, but your fish is likely to make a full recovery. That being said, sometimes these stresses and illnesses can cause permanent impairments to fish. This can include scarring, disfigurement, eyesight loss, or discoloration.

More than likely though, your fish will be back to normal in no time as long as the source of the problem is discovered and treated.

How To Keep Your Goldfish From Turning Black

It’s easy to keep your goldfish from turning black. A goldfish developing black spots is either stressed or sick and is not normal. As long as your fish tank is set up correctly, then you should never experience this phenomenon.

Here is how to ensure that your goldfish never turns any other color than it’s supposed to be!

1) Fully cycle your goldfish tank. Sadly, goldfish are considered beginner fish, which means that they are often exposed to beginner’s mistakes. The main mistake made is not allowing the aquarium to fully go through the nitrogen cycle. This causes fewer beneficial bacteria to properly convert ammonia to nitrite to nitrate, leading to toxic conditions when fish waste buildup becomes too much. This causes ammonia burns and nitrite poisoning and eventually death.

2) Use adequate filtration. Goldfish are messy fish, creating a lot of waste and leaving a lot of leftover food. Not to mention they regularly like to dig up plants and decorations and disturb the substrate! It is recommended to have a filter rated for at least 4x the size of the aquarium when keeping goldfish. A large filter will allow for more space for beneficial bacteria to grow, hopefully stopping a goldfish from turning black.

3) Regular tank maintenance. Most times, a good filtration system is not enough to keep up with the amount of fish waste and uneaten food circulating throughout the aquarium. Goldfish tanks need regular aquarium maintenance. This includes water changes, filter upkeep, and removing uneaten food.

4) Regularly test water parameters. It’s easy to look in the aquarium and think everything is okay. Many problems in the aquarium don’t become apparent until it’s too late. High ammonia levels can be a silent killer that is otherwise easily preventable. By the time your fish starts to develop black patches and ammonia burns, ammonia will have risen considerably. This can be easily prevented by using an ammonia test kit regularly.

5) Quarantine new fish. No matter what kind of fish you’re keeping, it is always a good idea to quarantine fish. This allows you to monitor fish health and behavior in a safe quarantine tank over the course of several weeks. Mainly, this will let you safely dose medications to a goldfish turning black without having to worry about killing your other fish or beneficial bacteria.

6) Provide a healthy diet. Though less likely to happen, goldfish can lose their bright colors from a poor diet. These fish are eager omnivores and need a variety of live, frozen, and freeze-dried foods. Because they’re considered beginner fish, many commercial foods are cheaply made with little nutrition. It is even recommended to get color-enhancing goldfish food to make your fish show their brightest colors without skimping on nutrients.

Final Thoughts

It can be scary to see your goldfish turning black. This isn’t normal and indicates that there’s a bigger problem in the system. Usually, this is the result of ammonia burns, nitrite poisoning, fin rot, or black spot disease. Black patches can also be the result of poor water quality or unrelated stress as well.

Luckily, water parameters can be easily fixed and both fin rot and black spot disease are straightforward to treat. The real solution to keeping your goldfish happy and healthy is setting up an adequately sized aquarium with good filtration and a finished nitrogen cycle.

by Mark

Mark is the founder of Aquarium Store Depot. He started in the aquarium hobby at the age of 11 and along the way worked at local fish stores. He has kept freshwater tanks, ponds, and reef tanks for over 25 years. His site was created to share his knowledge and unique teaching style on a larger scale. He has worked on making aquarium and pond keeping approachable. Mark has been featured in two books about aquarium keeping - both best sellers on Amazon. Each year, he continues to help his readers and clients with knowledge, professional builds, and troubleshooting.

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Why Is My Goldfish Turning Black - 7 Reasons Why (And How To Prevent It)
Goldfish are meant to be gold. They should never start turning black or developing black patches across their body or fins. A goldfish that is turning black is not a good sign. So why is my goldfish turning black? Either there is a problem with water quality or the fish is being affected by disease or illness.