How to Treat White Fungus on Corydoras?

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If you’ve ever owned Corydoras or still do, you know that they’re particularly sensitive to poor water conditions.

They can struggle with a variety of health issues because of that, but today, we’ll only discuss one – white fungus.

This condition comes with a multitude of symptoms, and you need to assess as many signs as possible to diagnose it correctly.

That’s because several health issues come with similar symptoms. The white fungus itself can suggest cotton wool disease, a normal fungal infection, or even Ich.

Given that the cotton wool disease is the most prevalent let’s discuss this one more in-depth.

What Does Cotton Wool Disease Looks Like?

The cotton wool disease isn’t a fungal infection but a bacterial one. The culprit is Flavobacterium columnare which is responsible for the oldest-known fish diseases.

The pathogen enters the fish’s body through any orifice it can find, including the mouth, gills, and even tiny skin tears.

The resulting disease (Columnaris) advances fast and comes with multiple symptoms, such as:

  • White, cotton-like patches around the fish’s mouth or gills
  • Discolored patches on the body, typically white, yellow, or orange
  • Scales falling off and fin damage that grows with time
  • Skin becomes thinner and slightly transparent at times, revealing the fish’s muscles underneath
  • A distinct white patch, sometimes wooly in appearance, present behind the dorsal fin (hence, the alternate name of the saddleback disease)

The discolored body patches can also become slightly fuzzy in appearance, making the condition appear fungal in nature.

This causes many aquarists, including more experienced ones, to misdiagnose the problem and resort to ineffective treatment.

This is a critical mistake, given the disease’s aggression, spread rate, and unhinged development rate.

What Causes Cotton Wool Disease in Corydoras?

There are 2 primary causes responsible for the cotton wool disease:

  • Improper water conditions – Combine massive aquatic bioloads with low-to-no maintenance, and you’ll be creating the perfect environment for Flavobacterium columnare to thrive. Corydoras are cleaner fish, but they can’t keep the environment clean and healthy on their own. You also need a good filtration system, a proper maintenance routine, and regular water changes to keep the catfish’s habitat fresh and clean.
  • Fish stressStressed fish have lower immune systems, which opens them up to infections and parasites. A healthy fish with a strong and active immune system is less prone to such issues. Fish stress relates to a variety of problems, including poor water conditions, improper diet and water parameters, aggressive tankmates, an inadequate substrate, an unfitting tank layout, etc. All fish require personalized aquatic ecosystems to thrive, and Corydoras are no different.

You can prevent the risk of your catfish contracting Columnaris disease by working on these 2 problems beforehand.

But once the infection is on, you need to act fast. The cotton wool disease advances and spreads faster than you might suspect, often with fatal consequences.

How to Treat Cotton Wool Disease in Corys?

If you notice any signs of white fungus on your cory(s), you’ve already started your race against time.

Remember, this disease is fast-acting and doesn’t require more than a couple of days to kill. Death can sometimes arrive even sooner, depending on your fish’s overall health state, age, etc.

Here’s what you need to do:

  • Quarantine – The first and most important step, given that most fish disorders are contagious. Columnaris is too, which is why quarantine is essential to contain the pathogen’s spread. Check all fish to identify and isolate the sick ones. The hospital tank should have similar water parameters to the main one to prevent the new tank syndrome, temperature or pH shock, or any other problems associated with the lack of acclimation.
  • Adequate medication – Nitrofurazone and Kanamycin. These 2 medications are generally great at addressing the cotton wool disease, although the dosing may vary depending on the fish’s needs. I recommend consulting your vet before going for medication treatment.
  • Pristine water quality – You want to perform a partial, minor water change of 5%-10% daily for the entire duration of the treatment. This is to keep water quality high and remove any traces of organic matter that could alter the water’s chemistry.
  • Water conditioner – Use a good water conditioner to decrease your fish’s stress and speed up recovery.

Pro tip – avoid salt. Salt is generally useful when treating various fish disorders because it aids in healing and calms the fish.

The problem is that you can’t use salt with Corydoras and other catfish species due to their lack of scales.

So, you shouldn’t use salt for your Corydoras during treatment. Some very small amounts might help, but it’s easy to add more than you should, hurting the fish in the process. It’s safer to avoid it altogether.

Is Cotton Wool Disease Fatal to Corys?

Yes, it is. The cotton wool disease is very aggressive and can kill the infected Corydoras in less than 48 hours.

This timeframe can easily drop to 24 hours if the affected cory has a weak immune system, is old, or is dealing with other disorders on the side.

This is why early detection and immediate treatment are necessary to contain and counter the disease effectively.

Is Cotton Wool Disease Contagious?

The cotton wool disease is highly contagious. If one fish has it, others have contracted it as well, although this may differ.

The pathogen is more likely to infect already sick or weak fish, while healthy ones may take more time to experience the same fate.

This is why I recommend immediate quarantine as the best approach in this situation. Isolating the sick fish will contain the disease, provide more control over the treatment, and prevent the pathogen from infecting the healthy hosts.

The Columnaris bacteria will eventually infect the healthy catfish, given sufficient time.

How to Prevent Fungus Disease in Corys?

I have 4 overarching prevention methods I want you to consider:

  1. Proper tank maintenanceVacuum the substrate regularly (as often as necessary), clean algae residues and fish waste, and perform weekly water changes. Your catfish need a healthy, clean, and fresh ecosystem to thrive.
  2. Avoid fish stress – Fish can get stressed for a variety of reasons, including aggressive mates, an improper substrate (sand is preferably or any fine-grained substitute), poor diets, etc. Stressed fish have weaker immune systems, which turn them more vulnerable to bacterial infections. Identify the cause of stress and remove it as soon as possible.
  3. Adequate diet – While they are apt scavengers, catfish still need a nutritious and well-balanced diet to remain healthy and healthy. Feed your catfish 2 times per day and mix their meals a bit. They require both animal protein and veggies for an optimal nutrient intake.
  4. Keep temperatures stable – Improperly high water temperatures are known to trigger the Columnaris disease. That’s because these bacteria thrive in higher temperatures above 80 F. Also, keep in mind that higher temperatures speed up the bacteria’s metabolism, resulting in a higher mortality rate and a lower time-to-kill.

If you’ve done everything in your power to prevent the disorder, but failed, quarantine the fish immediately.

If the fish doesn’t respond to treatment as it should, euthanasia is your only option.

Columnaris is simply too aggressive and contagious to allow the infected fish to return to the main tank.


Unfortunately, the cotton wool disease has a pretty high mortality rate due to its aggression and spread capability.

The disorder can aggravate fast and take over the entire fish population within days.

Prevention, early detection, and treatment are your best tools against this aggressive pathogen.

Author Image Fabian
I’m Fabian, aquarium fish breeder and founder of this website. I’ve been keeping fish, since I was a kid. On this blog, I share a lot of information about the aquarium hobby and various fish species that I like. Please leave a comment if you have any question.
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