White Fungus On My Oscar (What It Is And How To Treat It)?

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The white fungus you can see growing on your Oscar is a bacterial infection. The most common places for white fungus to grow on an Oscar are around the mouth, on the fins, or in open wounds. The fungus should be treated with an appropriate fish medication like Maracyn by Fritz Aquatics.

I have been keeping and breeding Oscars for almost 30 years. They are by far the fish I have dedicated more time to than any other fish. Over the years I have had my fair share of Oscars with infections or diseases. Provided we identify the disease and start treatment quickly, there is no reason almost all Oscars can’t be brought back to full health.

Oscars are often more than just another fish in our aquariums, they are our pets. When they get sick it can be distressing. Oscars can be especially susceptible to three diseases, Hole in the Head, Ich, and Fungus. In this article I answer the question What is the white fungus on my Oscar and how do I treat it?

What Causes The Fungal Growths?

Bacteria is always present in our aquariums. It is ‘good’ bacteria that we rely on to keep our aquarium water clean by converting the fish waste (ammonia) into a far less toxic form (nitrate). We couldn’t keep fish with good bacteria.

Bad bacteria on the other hand is always present too. This bacteria remains in our water, no matter how good a fishkeeper we are. The bad bacteria don’t pose a real threat to the fish, providing they remain healthy. Once a fish’s immune system becomes compromised, the bacteria is poised to take advantage.

A fish’s immune system can become compromised for a number of reasons, including:

  • Poor water quality
  • Overcrowding
  • Stress
  • Poor diet
  • Injury

Poor Water Quality

Although we usually have a filter running on our aquariums, the filter can only convert the fish waste into a less toxic form, known as nitrate. These nitrates continue to build up in our aquariums and the only practical way to reduce them is to change some water.

Removing a percentage of the aquarium water and replacing it with freshwater reduces the levels of nitrates and keeps the water fresh for the fish to live in. I have had great success using a Python Kit (this one on Amazon is the best value Ive managed to find)

When we don’t change the water often enough, the quality of the water goes down. The water quality can get so poor that the fish becomes stressed and prone to bacterial infection.

To test your aquarium water to see if there is ammonia, nitrite or nitrate present, I recommend using the Master Test Kit by API. I have found the test kit to be easy to use, fairly accurate, and cheap. I purchased mine from Amazon.


Overcrowding an aquarium can quickly lead to water quality problems. When you have a large number of fish in a single aquarium, the amount of waste the fish produce can overwhelm the filter, leading to a drop in quality.

Overcrowding can also result in fish ‘pecking’ at one another and fighting for space. This can lead to stressed fish and injuries which can, in turn, become susceptible to bacterial infections.


As discussed above, stress can cause a fish’s immune system to become compromised which will often allow bacteria to take advantage. Stress can be caused by poor water quality, overcrowding, bullying or by outside factors such as constant loud noises or children banging on the aquarium.

Poor Diet

Fish in our care are totally reliant on us to cater to their needs. Oscars are large, muscly fish that require a varied diet to satisfy all their nutritional needs. If we just feed them the same, low-quality flake or pellets every day, their nutritional need won’t be met.

Without the proper vitamins and minerals, Oscars can become weak and bacterial infections may take advantage.

If you are looking for ideas about how to vary an Oscar’s diet, check out this article I wrote titled 10 Unusual Foods Your Oscar Will Actually Enjoy.


Oscars can become injured either during transit or if they are attacked or bullied by another fish. A fish’s skin is its first line of defense against bacteria. Once that defense is broken, bacterial infections can quickly take hold.

How To Treat Bacterial Fungus In Oscars

There are a number of products on the market that are safe and effective against bacterial fungus infections in Oscars.

In the US, Maracyn by Fritz Aquatics is extremely effective. It has erythromycin as the active ingredient. It is safe to use on almost all fish in the hobby and it is also safe for aquariums that have plants or shrimp in it.

In the UK and mainland Europe, eSHa2000 (fungus and fin rot treatment) is the product of choice. More on eSHa2000 can be found in this article.

Whatever treatment you choose, ensure you test your water quality too. If your water quality is poor, your Oscars may remain susceptible to another bacterial fungus once this one is cleared up. I have found the API Master Test kits to be a very effective way to test water quality.

How To Prevent Bacterial Fungus In Oscars?

As I have mentioned above, bacteria are ever-present in our aquariums. We can’t do anything about that. What we can do is give our Oscars the best chance of not getting a bacterial infection in the first place.

  • Keep water quality good – Carry out regular water changes and test your water frequently so you can be sure the quality is good. Try not to pollute the water in the first place. Over feeding is a classic way to reduce water quality quickly.

  • Don’t overstock your aquarium – The more fish you have, the greater the chances the filter won’t be able to cope and the higher the chances the fish will be ‘pecking’ at each other. Aquarium stocking is always and has always been a hotly debated topic. There are no hard and fast rules for how many fish you can have in an aquarium, just try to use sensible judgement and monitor water quality. Choose your Oscars tank mates wisely!

  • Feed a quality diet – Like with us humans, the better the food we eat, the stronger and healthier we are. A strong, healthy Oscar will be much better prepared to fight off a bacterial infection. I like to feed my Oscars Bug Bites by Fluval (more on Bug Bites in this article I wrote titled Are Fluval Bug Bites Good For Fish?). They are high in protein and Oscars seem to love eating them

My Final Thoughts On White Bacterial Fungus On Oscars

Bacteria, both ‘good’ and ‘bad’ will always be in our aquariums. There is nothing we can do about it, and we couldn’t actually even keep our Oscars without the help of some bacteria. What we can do as responsible Oscar keepers is give our fish the best chances of not getting a bacterial infection by keeping their water clean, feeding them a quality diet, and making sure we keep factors that may stress our Oscars to a minimum.

About the Author

I’ve been keeping, breeding, and showing tropical fish for nearly 30 years. Over that time I’ve done it all! I’ve had great success and I’ve made some really foolish mistakes (like the time I bought an Asain Walking Catfish). Read more…
Richard James

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