Why Is My Oscar Fish Turning Black? (Spots & Patches)

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It can be disheartening to see the moment your new Oscar fish has developed black patches or spots on their scales, fins, and tail. I remember when I first saw it and begged it not to die. As time passed, I learned what may cause Oscar fish to turn black and how to overcome the issue.

Oscar fish tend to develop black patches due to environmental factors, including stress, injuries, and ammonia spikes. These will cause black marks to develop on the Oscar fish’s scales, fins, and tail. That is in contrast to black spots, which are commonly related to a parasitic infection.

As we move forward, I will take you step-by-step through the possible scenarios that will result in an Oscar fish turning black and what we can do to prevent them from deteriorating. Bear in mind that I’ll distinguish between black spots and black patches, as the two have different causes.

Also Read: Stress In Oscar Fish

Why Is My Oscar Fish Turning Black?

Oscars are large fish. Black markings on an Oscar are not always a cause for concern because you can find these creatures in various colors. In fact, some Oscars are black with red and orange markings.[1]

Black spots and patches should only concern you if they are new; that is to say, the Oscar fish did not have them before. 

This shouldn’t necessarily worry you. The reasons that can cause Oscar Fish to turn black will vary. Some of those reasons are perfectly innocent, while others can lead to the death of the fish. They include:

1. It’s Part Of The Oscar Fish’s Genes

First of all, you have to realize that it is perfectly normal for Oscars to change their colors.[2] The creatures won’t develop their final colors and patterns until they reach adulthood. But even after they reach adulthood, the colors of an Oscar can change as it transitions into old age.

In many cases, older fish will manifest duller and less vivid colors than their younger counterparts. On occasion, an Oscar’s genes may compel the fish to develop spots and patches as it transitions into a different stage of its life cycle.

Keep in mind that, in the absence of a drastic transition in age, you may also observe color changes that correspond to the Oscar’s mood. The colors are much brighter in an angry Oscar. 

If it had dull spots and patches, you might notice them for the first time during this period because of how vivid they have become.

Also Read: Why Is My Oscar Fish Turning Gray?

2. Your Oscar Caught Black Spot Disease

People associate this disease with Surgeonfish, but it can affect other species as well.[3] The illness is caused by Paravortex turbellaria flatworms that burrow into the skin or gills of the fish where they feed on cell fluids.

People call this disease ‘Black Ich’ because black spots appear all over the fish’s body. But unlike the white spots that appear when a fish has ich, the black spots on an Oscar will move. This is because those black spots are worms, and the worms are mobile. 

An Oscar with black ich will respond by crashing into and rubbing against objects. Over time, the creature will become lethargic. If the worms attack the gills, the Oscar will pant and gasp for air.

3. The Oscar Fish Is Stressed

Like all fish, Oscars hate stress. It makes them weak and lethargic, not to mention vulnerable to illnesses like black spot disease. Stress has various sources, including poor water quality, the wrong parameters, and aggressive neighbors.

It isn’t uncommon for fish to change color in response to stress. Discus fish are one example. They can turn black because of stress.[4] Other fish turn white, losing their marks, lines, and spots. 

You won’t know how your Oscar will respond until it is exposed to stress. Some Oscars may grow pale, others may become darker, and yours could develop spots.

Also Read: Why Do Oscar Fish Shake?

4. Your Oscar Fish Is Injured

Aquariums are safer than conventional water bodies in the wild. Your Oscars don’t have to worry about being eaten by predators or going hungry. However, aquariums are not perfect. 

Aquarium fish are prone to injuries. They may clash with aggressive tankmates or collide with objects or accumulate burns from the heater. If the Oscar has round spots all over its body, you can’t blame them on injuries. 

However, if the creature has developed randomly shaped patches, you are probably looking at the final result of continuous bruising and injury. 

5. The Oscar Fish Developed Fin Rot

If the color change is restricted to the fins and tail, the Oscar probably has fin rot. When a fish contracts fin rot, the fins and tail become black and tattered. 

After a while, they may fall off. An Oscar with fin rot will become lethargic, eating less and spending more time at the top of the tank.

6. The Ammonia Levels Are Too High

Ammonia is one of the most dangerous substances in a tank. It won’t stop at compromising the immunity of the fish. It will cause physical harm in high concentrations, burning the gills and producing streaks and patches on the Oscar’s skin.

On the whole, the black patches are a good thing. You can’t see the ammonia burns on a fish. The streaks and patches only form when those burns start to heal.[5]

This will alert you to the presence of ammonia. If your Oscar looks burnt, you should act quickly to bring the ammonia concentration under control. 

7. The Water Conditions Aren’t Suitable For Oscars

Poor water conditions can force a fish to turn black by inducing stress, lowering the immunity of the fish and making them more susceptible to illnesses like fin rot, and permitting ammonia levels to spike.

Oscars are not immune to poor water conditions. Whether they develop spots or patches will depend on the ailment, they eventually contract.

Ammonia burns will cause patches. Stress is more likely to produce spots, especially if the Oscar has genes that predispose the creature to such alterations in appearance. 

How Do I Treat Black Spots On My Oscar Fish?

If your Oscar has black spots, you have to consider the possibility that the change is genetic. If the Oscar fish hasn’t manifested any signs of ill health or distress, you can leave it alone.

On the other hand, if the creature is clearly sick, you should take steps to treat the factors behind the black spots. More often than not, illnesses such as black spot disease and fin rot are to blame:

1. Fin Rot

Medical experts have different opinions regarding the cause of fin rot. They agree that bacteria are to blame, but they disagree on the exact bacteria. The candidates include Pseudomonas spp, Cytophaga spp, Flavobacterium Columnarae, etc.

As an aquarist, you don’t have to identify the bacteria that caused fin rot in your Oscars. Just recognize that bacteria are to blame, making antibiotics the most apparent solution. Experiment with oxytetracycline, chloramphenicol, tetracycline, and the like.

I personally recommend the API MELAFIX Bacterial Infection Remedy (link to Amazon). Use one tablespoon for every ten gallons of water. Repeat the process for one week and look for signs of improvement. This product only works in freshwater aquariums.

To be on the safe side, you should perform a water change before you treat the Oscar. Some Oscars will regrow their fins and tails in the long run. Others won’t. You won’t know until you treat them. You can enhance their recovery rate by adding some aquarium salt.[6]

Also Read: Oscar Fish Fin & Tail Rot

2. Black Spot Disease

Black spot disease is not that difficult to treat. The parasites that cause black ich enter the tank via new fish or snails. The larvae of the parasites will develop in the liver of a snail. Eventually, they will leave the snail and burrow into the skin of a fish.

Black ich irritates the skin, but it rarely kills. In most cases, you can combat the disease by removing the snails, ending the life cycle of the parasites.[7] You can apply mild antibiotics to treat any bacterial infections that may occur due to the activities of these parasites.

Don’t forget to clean the tank. Because the worms feed on the organic matter in the substrate whenever they leave their hosts, you can starve them by vacuuming the substrate and keeping fish out of the tank for a few months.

You can also use products like Seachem ParaGuard (link to Amazon) to fight the parasites directly. Use one five m/L (one capful) of ParaGuard for every 10 gallons of water. Repeat the process daily until the fish shows no signs of stress. 

Also Read: Black Spots On Oscar Fish

How Do I Treat Oscar Fish With Black Patches?

You are more likely to encounter black patches in poorly maintained tanks filled with aggressive fish, which means that you have to improve the conditions in the tank before the Oscar fish can recover. Keep the following in mind:

1. Preventing And Treating Injuries

If your Oscar’s black patches are the result of injuries and bruising, you can take the following steps to protect them:

  • Tankmates – Pair Oscars with friendly fish such as plecos, parrotfish, clown loaches, and jaguar cichlids.[8] Oscars are large and aggressive enough to survive in a tank with cichlids. But if the Oscar or the cichlids are too violent, the Oscar can live alone.[9] It won’t get lonely. 
  • Handling – Handle the Oscars carefully. The wrong net can detach scales. Use fine netting that is less likely to harm them.
  • Tank Size – Keep the Oscar in tanks of at least 75 gallons. Avoid overcrowding at all costs. A crowded tank invites aggression and violence.
  • Substrate – Avoid sharp sand and coarse gravel.[10] They can damage Oscars with ich that rub against the substrate.
  • Vet – Consult a vet where necessary. They can determine whether the injuries and bruises require antibiotics.[11]

Also Read: Oscar Fish Tank Setup

2. Adjusting The Parameters For Oscar Fish

I highly suggest maintaining the correct parameters. Oscars require a temperature of 77 degrees F and a stable pH of 7.2.[12] The water should be soft, ranging from 5 to 20 dH. Avoid items like coral and limestone that people use to raise the pH. 

The proper parameters prevent stress and create a conducive environment that aids the healing process for sick or injured Oscar fish.

Toxins like ammonia and nitrites should be kept at 0 ppm, while nitrates are below 20 ppm. I personally measure these using the API Water Test Kit (link to Amazon). This bundle is highly cost-effective since it lasts for about eight hundred measures.

Also Read: What Is The Best pH Level For Oscar Fish?

3. Dealing With Ammonia Spikes

You combat ammonia by adding water conditioners. But you should only rely on conditioners during emergencies. They can neutralize the ammonia within minutes. 

In an ideal scenario, you should prevent the ammonia from spiking in the first place by performing regular water changes and removing organic debris from the tank. I personally replace 15 to 20 percent every week.

4. Maintaining A Clean Environment

Keep the tank clean. Check the filter to ensure that it is working. Make sure the filter’s strength suits the size of the tank; the bigger the tank, the more powerful the filter. You should also perform regular water changes. This will keep ammonia levels under control.

5. Adding Plants And Decorations

Add plants and decorations to tanks with stressed fish. The presence of hiding places puts Oscars at ease, especially if they live in tanks with poor conditions and predators. Some suitable plants for an Oscar tank include duckweed, frogbit, and hornwort.

Also Read: 11 Plants Your Oscar Fish Will Love In Their Tank


Oscar fish usually turn black due to injuries and bruising, but they might also be affected by ich and black spot disease. These conditions are easy to treat if you follow the proper protocols.

If you keep the water chemistry under control, provide hiding places for your fish, and maintain a clean environment, you should have healthy Oscars for years. 

Don’t forget that Oscar fish can change color with age; some will become pale, while others will turn darker. If everything is okay with the water and the fish appears healthy and energetic, you can blame the blackening on genetics.


  1. https://animaldiversity.org/accounts/Astronotus_ocellatus/
  2. https://amazingcichlids.com/oscar-fish-changing-color/
  3. https://www.thesprucepets.com/black-spot-or-tang-disease-2920812
  4. https://be.chewy.com/aquarium-fish-stress/
  5. https://www.aquariumsource.com/goldfish-turning-black/
  6. https://small-pets.lovetoknow.com/fish/oscar-fish-diseases
  7. https://www.angelfire.com/blues/fish_problems/BLACKSPOT.html
  8. https://smartaquariumguide.com/best-oscar-fish-tank-mates/
  9. https://aquagoodness.com/can-oscar-fish-live-alone/
  10. https://be.chewy.com/aquarium-fish-injuries/
  11. https://www.cuteness.com/article/heal-injured-fish
  12. https://modestfish.com/oscar-fish-care/