Why Do Discus Fish Turn Black? {Symptoms & Treatment Explained}

Is your Discus Fish turning black in certain areas and you’re wondering what went wrong? Could there be more than one explanation and what can you do about it?

This article is dedicated to helping you understand why your discus fish is turning black including symptoms and treatments.

Why Do Discus Fish Turn Black? Poor water quality, small tank sizes, stress from loneliness, bullying, poor acclimation, disease and infection are common reasons why Discus Fish start to turn black. Also make sure the substrate and background is light in color. 

What Symptoms Accompany A Discus Fish Turning Black?

A Discus fish is beautifully colored and popular for its appearance. Once you notice it turning black, there must be something going on.

Look for the following symptoms:

  • Black spots peppered around the body
  • Heavy breathing
  • Glass surfing (move up and down the tank)
  • Hiding in the corners or planted areas
  • Leaning to a side
  • Loss of appetite
  • Lethargy
  • Clamped fins
  • Slime coating
  • Excessive mucus
  • Scratching against objects
  • Erratic swimming patterns
  • Twitching
  • Fin rot
  • White spots
  • Cloudy water from excess mucus

If you see some of these symptoms that accompany the darkening appearance of your Discus Fish, you must act now to treat the water chemistry first and foremost.

What Causes A Discus Fish To Turn Black?

Is your Discus Fish turning black in color or are the black areas more like spots? It’s unfortunate that either is occurring, but we should try to rule out infections from black spots such as fluke disease.

Gradual overall color changes or losing that beautiful blue color may indicate other behavioral or water chemistry factors such as:

  • loneliness
  • stress
  • poor water parameters
  • incorrect or rushed acclimation 
  • aggressive tankmates
  • overcrowding 
  • insufficient tank space

Since Discus Fish reach around 4.5-6 inches in length, it’s recommended to keep them in tank sizes of 55 gallons or larger.

Keep in mind that you should keep them in groups of 4 or more and 55 gallons is the least amount of space to consider.

What Are The Best Water Parameters For Discus Fish?

Discus Fish enjoy the following water parameters that can help maintain its healthy and colorful appearance:

  • pH: 6.0-7.0
  • Temperature: 82-89°F
  • Water hardness: 18 ppm to 70 ppm
  • Chlorine: 0.001 ppm to 0.003 ppm
  • Ammonia: 0 ppm
  • Nitrite level: 0 ppm
  • Nitrate: less than 10 ppm

Fluctuations and spikes from poor filtration, excess debris, detritus in the substrate and overall cloudy water can affect the outward appearance of your Discus Fish, but also introduce internal illnesses and infections as well.

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Is My Discus Fish Lonely?

If you are keeping less than 3 Discus Fish, you may have the reason why your Discus Fish is turning black right in front of you.

Yes. Discus Fish get lonely and prefer to be kept in groups of 4-6 or more. The amount of space at 55 gallons or larger must also be in place to make sure your Discus Fish is not stressed.

Companionship is very important to Discus Fish. Keeping one a trophy Discus Fish for its gorgeous colors will soon turn to a darkening complexion or illness that could end its life prematurely.

Why Did My Discus Fish Turn Dark Right Away?

You may have rushed the acclimation process when introducing your Discus Fish to your tank. The “drip acclimation” approach is the better method that takes more patience with best results.

Fixing the temperature to match the water in the bag where your Discus Fish anxiously awaits its release, is simply not enough.

In a matter of days, you may notice the stress taking hold and your Discus Fish turning black as a result.

Is My Discus Fish Sick?

If the first sign you are noticing is that your Discus Fish is turning black in color, there could be more symptoms to follow.

A lack of appetite or lethargy where your Discus Fish loses interest in doing anything could occur.

The following bacteria, fungus or parasites must be researched separately to find out if they may be responsible for what you are seeing in terms of a darkening complexion:

  • Spironucleus spp
  • ichthyophthirius multifiliis
  • Aeromonas hydrophila
  • Fusarium solani
  • F. moniliform
  • F. oxysporum 

These types of inflictions may end up causing harm to your Discus Fish and fellow tankmates.

What Is the Minimum Tank Size For Discus Fish?

Your Discus Fish is sure to become stressed if the tank is too small. One Discus Fish theoretically could be kept in a 10 gallon tank, but loneliness will result in stress and darkening in color.

You will need at least 4, preferably 5 Discus Fish to keep them safe and secure in their natural desire to increase their strength in numbers.

In this case, you should only get into the hobby of keeping Discus Fish if you have a tank that is 55 gallons or larger to fit at least 4-6 of them together.

How Do You Cure Black Colored Discus Fish?

If you know that loneliness is not the case and your tank is at least 55 gallons in size, then you may need to treat a possible illness leading to a black coloration on your Discus Fish.

  • Use a quarantine tank if you can.
  • Test the water in the main tank for optimal parameters. 
  • Monitor all other fish for signs of illness (cloudy eyes, lack of appetite, lethargy).
  • Add air stones (optional) or increase water circulation. 
  • Vacuum the substrate of the main tank.
  • Perform a large (50-75%) water change gradually. 
  • Add one tablespoon of aquarium (pre-dissolved in aquarium water first) per 5 gallons of water gradually. 
  • Speak to a vet or someone at the local fish shop about Metronidazole or Dimetridazole or follow instructions on the bottle. 
  • Consider using Melafix for possible bacterial infections. 

How Do I Prevent My Discus Fish From Turning Black?

The following tips are preventative measures to make sure your Discus Fish doesn’t succumb to changes in its coloration from blue to dark blue or black.

  1. Keep optimal temperature and water chemistry for Discus Fish.
  2. Perform weekly water changes at 15-30%.
  3. Vacuum the substrate.
  4. Keep a light colored substrate.
  5. Ensure that you keep at least 4-6 of them at a time.
  6. Use a 55 gallon tank or larger.
  7. Remove leftovers after 3-5 minutes of dropping in meals.
  8. Feed a variety of foods and not just pellets or flakes.
  9. Use drip acclimation and a possible quarantine tank for every new Discus Fish.
  10. Observe for other signs of illness from white spots, to cloudy eyes or excessive hiding.


We hope your Discus Fish that has turned black recovers with the right measures put into place from the information we have provided in this article.

It’s unfortunate that this has occurred, but it’s common and can be cured in many cases. It’s preventable in the future and we wish you the very best in your journey through this challenging, but very rewarding hobby.

Thanks for visiting HelpUsFish.com and see you again soon!

Brian Arial

Brian Arial has kept fish for leisure and worked with fish stores for most of his life. He enjoys writing and caring for aquariums and ponds.