Why is my Koi Turning Pink? (With 4 Quick Solutions)

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As a fish owner, quite frequently, I noticed changes in my Koi’s coloration. For example, a few times, I saw that my Koi fish are turning pink. As I studied the phenomenon, I found a few ways to distinguish an underlying disease from a natural coloration process. To ensure that you take the right approach, I decided to share my conclusions.

Koi fish tend to turn pink due to an underlying disease, such as Bacterial Hemorrhagic Septicemia, Fin Rot, and Pseudomonas infections. However, Koi fish may also become pink due to ammonia spikes, color-enhancing diet, stressful conditions, and sunburns secondary to direct sunlight. 

As we move forward, I will share four essential steps that should treat the pinkish coloration’s underlying conditions. I will also show you the typical signs of a dying Koi fish to determine your pet’s general state.

Why is my Koi Turning Pink?

Koi fish will turn pink for any number of reasons. Sometimes, they are closer to red than pink. Other times, they are more pinkish than red. Regardless of the color, the appearance of streaks, scars, or spots on your koi fish should concern you because some of the potential causes are quite serious. 

They include:

1. Underlying Disease

Even though koi fish are hardy, they are still susceptible to a variety of diseases. Those could influence your fish’s general state and colors. Sometimes, a sick koi will gradually turn pink. Common diseases include:

  • Bacterial Hemorrhagic Septicemia – Causes streaks and open sores to appear all over the Koi’s body.[1]
  • Pseudomonas Bacteria – Produces open sores of a pinkish or reddish color.
  • Carp Pox – Produces a pinkish flush.[2]
  • Fin Rot – If the fins and tails are the only parts of the body displaying this discoloration, the Koi fish might have fin rot. The fins of koi fish with fin rot will develop fuzzy growths.[3]
  • Injury – Sometimes, pinkish/reddish ulcers develop on a koi’s body because bacterial infections have invaded the site of a physical injury that the fish recently suffered. That is why aquarists are encouraged to treat wounded koi fish immediately.

3. Inappropriate Water Conditions

Koi fish have a coating of slime that is supposed to protect them from harm. However, low water conditions can degrade the coating, leaving the creature vulnerable to bacterial infections.

Koi fish require specific parameters in their water. The wrong conditions in their tank can lead to unexpected symptoms such as a pink flush.[4] That includes too high temperatures, the wrong pH and hardness, and a small, overcrowded tank.

I also recommend paying close attention to the nitrates, nitrites, and ammonia. If your Koi fish is turning pink, one of these toxins is probably too high. If the creature is kept in a pond outside, you should consider the impact of factors like airborne pollutants and acid rain.

The ideal water parameters for koi fish are:

  • Water pH: 7.5 to 8.[5]
  • Temperature: 59 to 77 degrees F.[6]
  • Ammonia, nitrates, nitrites: should be around 0.

3. Color-Enhancing Food

Koi fish are popular because they are so colorful. However, some aquarists will attempt to enhance the appearance of their fish by giving them color-enhancing food. This practice is, for the most part, acceptable.

However, if you give your fish color-enhancing foods like Spirulina in massive quantities, you could produce drastic changes in their appearance. For instance, white areas on their bodies will become pink.[7]

4. Stressful Conditions

Stress is one of the most common causes of discoloration in Koi fish. Familiar sources of stress in Koi fish include low water quality, an unbalanced diet, toxins and pollutants in the water, overcrowding, and aggressive neighbors, to mention but a few.

Besides changing color, stressed fish will jump out of the water, rub against objects in the tank, hide, lie on their sides, etc.[8] If your Koi is stressed, it will also be more susceptible to diseases, such as fin rot and bacterial infections. And as was mentioned, those can induce pinkish colorations.

5. Direct Sunlight

Koi fish have a slime coating that is supposed to protect them from elements like the sun. But that doesn’t make the koi fish immune to sunburn, mostly if it lives in a pond. Some sunburns are more severe than others:[9]

  • Minor sunburns will produce a mild pink rash. You should expect additional symptoms such as lethargy and loss of appetite.
  • Moderate sunburns will result in a deep red color. Lesions may also appear.
  • In the most challenging situations, the fish will develop blisters and ulcers. It may even die.

6. Genetics

Some koi fish naturally change color as they mature. If your fish has developed a reddish or pinkish hue, but it is entirely healthy, you have nothing to worry about. The creature is merely manifesting the color it was always meant to have.

That is particularly true for koi fish that are growing in size. The young ones typically appear pale and sometimes even white. However, as the weeks pass by, the young Koi will grow and present more intense colors (including yellow, orange, red, and pink).

How to Treat Koi Fish that Turned Pink?

If your Koi is turning pink and features ripped fins, sluggish behavior, or loss of appetite, it is most likely sick. That case requires your attention. However, you have to match the treatment to the cause of the discoloration, for instance:

1. Treating Diseases

If you suspect that your Koi is sick, my first advice is consulting a vet. They will identify the disease ailing your koi fish and then prescribe an effective treatment. More than likely, they will recommend oxytetracycline (in the food) and Forma Green (in the water) for Aeromonas Hydrophila.

Pseudomonas Fluorescens, on the other hand, responds to Kanamycin and Neomycin (in the food). You can also use Forma Green (Water). Koi fish with blisters that have been imputed to Aeromonas Salmonicida can be treated with Furazolidone.

Aquarium salt is often encouraged because some viruses and bacteria hate it. The salt will also enhance the effectiveness of the drugs you have chosen to deploy. That is on top of encouraging the regeneration of the Koi’s slime coating.

Depending on the bacterial strain in your tank, you should think about raising the temperature to 82 degrees F. Salmonocida, for instance, cannot survive at temperatures that high.[10] That temperature is slightly too warm for koi fish, although it may help overcome infections in the short term.

Wounds should be cleaned and treated with antibiotics like Neosporin (ointment). Some ulcers will respond better to iodine. However, none of these treatments matter unless you maintain a clean tank, free from bacteria and parasites.

Additionally, it would help if you didn’t forget to quarantine the diseased fish. Even if the creature has an illness like fin rot, which isn’t contagious, the antibiotics used to treat fin rot could negatively affect the tank’s biological filtration capacity.

2. Improving Water Conditions

Try to maintain the best possible conditions in the tank. Koi fish are large, reaching 36 inches in size. That is why you are encouraged to keep them outside in ponds. They need at least 50 gallons of water per fish.[11]

Any pond used to store them should be at least 6 feet deep. That will allow the koi fish to survive in the winter even when the top of the pond freezes. Koi fish also require a pH ranging from 6.0 to 9.0 and temperatures ranging from 74 degrees F to 86 degrees F (in a pond).[12] 

In an aquarium, they prefer temperatures of 65 to 75 degrees F. It would be best if you also changed their water regularly. If they turn pink, you should change their water immediately because they are most likely facing a spike in toxins.

I also recommend testing your water for toxins. For that, I personally use the API Aquarium Test Kit (link to Amazon). That bundle will measure your pH, nitrates, nitrites, and ammonia within minutes with high accuracy. I love that kit because it lasts forever and feels the most cost-effective choice.

Regardless of the Koi’s environment, the ammonia levels should be around zero. That is true for both aquariums and ponds. If you’ve noticed an ammonia spike, you should perform more regular water changes, or possibly install a better filter.

I recommend installing a filter that can process all the water at least three times in an hour. Also, consider investing in bacterial supplements that can control the waste in the tank. I personally use the API MELAFIX Fish remedy (link to Amazon)

A koi fish living in a well-maintained tank is less likely to contract illnesses resulting from bacterial infections and parasites. That could also reverse your Koi’s pinkish coloration since discoloration is merely a symptom of a more severe condition.

3. Lowering Stress

The easiest way to treat stress in koi fish is to maintain a conducive aquatic environment with the right parameters. You should also take steps to cure the koi fish of any diseases that might be ailing them.

I also recommend eliminating predators. If they are housed in a pond, you can protect them from birds by either moving them to an indoor tank or using deterrents like nets, alarms, and sprinklers.

Don’t forget to add hiding spots. That includes plants like water-lilies and hornwort. Those will keep your Koi out of sight and are incredibly beneficial against harassers. Also, consider installing a few humanmade shelters at the bottom of the pond.

4. Treating Sunburns

Minor sunburns can be treated by giving the koi fish shade. However, if the sunburns are severe, your fish probably requires antibiotics. They might also need salt baths. The best choice would be consulting a vet; they will recommend some efficient medicated dips.

Koi fish that are recovering from sunburns should be fed high-quality meals. The most popular choice is probably the Tetra Pond Koi Vibrance (link to Amazon). They need all the nutrients you can give them. The creatures are omnivorous. They can eat insects, algae, zooplankton, and commercial items with high protein content.

How do You Know if a Koi Fish is Dying?

Koi fish that are dying show loss of appetite and inactive behavior. You may also notice behavioral changes, such as erratic swimming, sluggishness, shyness, and air gasping. A dying koi will also have clamped-in fins, and it may lie motionlessly at the bottom of the tank.

Some fish die quite suddenly, without warning. Nevertheless, that isn’t the case with Koi fish. They display plenty of warning signs that will alert you to the fact that they might be approaching the end of their lives.

Some unique signs of a dying koi fish include:

  • Jumping – If your koi fish keep jumping, you should take that as a sign that they are greatly distressed. If your tank doesn’t have a cover, the koi fish could jump to their death.
  • Gasping – Fish that are gasping for air at the top cannot breathe. If you fail to remedy their situation, they will eventually die.
  • Lethargy – Some dying koi fish become highly inactive. They either swim at a slower pace than usual or spend all their time at the bottom, lying in place. That is a sign that they are too weak to move. It could also mean that they are at death’s door.
  • Ulcers – Koi fish whose bodies are covered in open sores and ulcers are not a pleasant sight. But their appearance could also mean that they picked up a disease that is in the process of killing them.
  • Weight loss – You should pay close attention to koi fish that have lost an alarming amount of weight, particularly if they have also stopped eating. That is a sign that they are suffering from a severe, life-threatening illness.


Some koi become pink due to genetic factors. That means that the Koi is destined to change its colors as it grows, and sometimes that includes pink. You typically see that in young Koi that were pale or white beforehand.

However, if your Koi seems sluggish, swims differently, or frequent the bottom sections, a pinkish coloration could be a sign of a disease. In that case, isolate the fish from its companions and consult a vet for further treatment.


  1. https://www.nationalfishpharm.com/koipond/bacterial_disorders.html
  2. https://koi-care.com/koi-diseases-treatments/
  3. https://animals.mom.com/goldfish-ripped-tail-4408.html
  4. https://www.angelkoi.com/copy-of-aeronomas-bacteria-1
  5. https://animals.mom.com/ph-level-safe-koi-pond-10843.html
  6. https://www.hartz.com/habitat-needs-maintenance-outdoor-koi-pondfish/
  7. https://modestfish.com/best-koi-food/
  8. https://www.stillmanlandscape.com/how-to-tell-if-your-koi-are-stressed/
  9. https://pondinformer.com/koi-sunburn-treatment/
  10. https://hanoverkoifarms.com/koi-symptom-diagnosis/
  11. https://www.petsmart.com/learning-center/fish-care/koi-care-guide/A0017.html
  12. https://www.fishkeepingworld.com/koi-fish/