Cotton Wool Disease in Aquarium Fish (Causes, Symptoms, Treatment)

Cotton Wool Disease is a common bacterial disease in freshwater fish. It causes fluffy, cotton-like growth on the infected area of the fish.

Aquarists should make sure the disease is treated in its early stages as it can be extremely harmful.

Below is a comprehensive guide on cotton wool disease in aquarium fish.

Causes of Cotton Wool Disease in Aquarium Fish

Cotton Wool Disease is caused by a bacterium (Flavobacterium columnare) that is common in almost every fish tank.

These particular bacteria aren’t dangerous in normal circumstances. However, it can trigger the spread of cotton wool disease because of certain factors.

Weak Fish

Even if you have a pristine fish tank with proper water conditions, the cotton wool disease may still spread among the fish, especially among the weaker ones.

Weak fish can either be older fish or those who have an injury.

They have a weakened immune system and are more prone to contracting the disease. Once infected, they can easily spread it to others.

Water Quality

If your fish tank has poor water quality, the bacteria will multiply and will start infecting the fish. Water contamination can happen due to a lot of reasons.

Water quality drops when there is a lot of organic waste. This can be leftover food or waste produced by the fish.

If left unchecked, the accumulation of waste can rapidly lower the water quality.

Close Proximity with Infected Fish

Cotton Wool Disease has a high infection rate and can easily contaminate other fish in the fish tank.

There are certain bacteria strains that are far more dangerous than others.

The ideal solution to prevent the spread of cotton wool disease is to separate all the infected fish into a different tank.

Isolation works best whether you have one infected fish or several.

Symptoms of Cotton Wool Disease

A fish suffering from cotton wool disease shows several symptoms that are instantly recognizable.

However, many aquarists have trouble differentiating between fungal infections and cotton wool disease because of the similarity between the two.

One of the most visible signs of cotton wool disease is the fluffy cotton-like growth on their skins.

This may be found near the mouth, on the fins, or as patches on the body.

The skin around the bacterial growth becomes discolored. Usually, the colors are white, gray, or black. You may also see an excess amount of mucus on the fish’s skin. This is another symptom of the cotton wool disease.

Cotton Wool Disease also affects the behavior of its host. The fish will stop eating its food and will exhibit signs of lethargy.

It might swim slowly near the water surface or the tank floor.

If the disease isn’t treated quickly, the whole body of the fish will start decaying, making it difficult for the fish to swim. It might even prove to be fatal for the fish.

Cotton Wool Disease Diagnosis

Cotton Wool Disease and fungal infections look quite similar; hence it is important to have a proper diagnosis. This procedure is to be done by a vet.

When diagnosing the fish, a small sample of the fish will be tested under a microscope.

Cotton Wool Disease will make the bacteria shape into small haystacks.

In case it is a fungal infection, the hyphae (filaments of the fungus) will remain distinct.

It will be necessary to identify the bacteria strain and, consequently, the antibiotics needed to treat the disease.

By no means should the antibiotics be self-assigned. It’s best to consult a vet. Wrong antibiotics can damage the fish’s biological filter system and can cause serious health risks.

Cotton Wool Disease Treatment Methods

Once Cotton Wool Disease has been identified, there are several treatment methods that you can use to cure the disease.

Salt Treatment

Using aquarium salt is a highly effective way to treat cotton wool disease. This is because the bacteria thrive in freshwater and have a very low resistance to saltwater.

You need to remember your fish would be freshwater. Hence, when putting the aquarium salt, don’t put excessive amounts of it since it can disturb their balance.

When doing the salt treatment, you should add a tablespoon of aquarium salt to a gallon of tank water to create a salt bath.

Put the infected fish in the salt bath for 5 minutes and put it back in its tank.

If you have fish that can survive in brackish water (mollies, for example), you can also add a tablespoon of aquarium salt to five gallons of tank water.

This has slower effectiveness, but it will still help the fish in their recovery process. The salt will directly attack the bacteria.

Also read: Can You Treat Fin Rot with Salt?


Medication is another method of treating Cotton Wool Disease. It is best to give the medication as early as possible to make sure the infection isn’t spread.

Tea tree oil has amazing antimicrobial properties, which make it an effective solution for treating cotton wool disease.

API Melafix is one such item that is excellent for this purpose.

API MELAFIX Freshwater Fish Bacterial Infection Remedy 16-Ounce Bottle

You may also go for antibiotics like Furan-2. Furan-2 specifically goes for the cotton wool disease bacteria. Moreover, it also cures spots and sores on the fish’s skin.

Is Cotton Wool Disease Fatal for the Fish?

It is extremely important to make sure the disease is treated as soon as possible. Otherwise, it can prove fatal for the fish if it has been allowed to grow unchecked.

Cotton Wool Disease progresses from having a cottony growth on the skin to crippling the fish forever.

This usually happens if the disease has targeted the fin. In such cases, the fish will have a lot of difficulties swimming around.

If the fin is infected, the bacteria will start decaying the fin, right down to its basal cells.

Because of this, the regrowth of the fin is prevented even after the disease has been treated.

At later stages, the fish will be unable to use its infected fin and will become limp.

Considering the severity of the issue, aquarists need to make sure cotton wool disease doesn’t target the mouth, eyes, and gills of the fish.

How to Prevent Cotton Wool Disease in Fish

The best way to make sure cotton wool disease doesn’t affect your fish is to clean your tank rigorously. Make sure your cleaning process is thorough and regular.

You will also have to keep a strict lookout for any deviant behavior among the fish. Look for signs like slow, erratic swimming or not eating anything.

The moment you notice something different, do address it quickly. Cotton Wool Disease has a fast infection rate and can spread out through the fish tank very easily.

You should make sure your fish are getting all the nutrients that keep them healthy and boost their immune system. They should also be given enough space.

Keep checking the water quality regularly. The fish tank water should have the right temperature and pH level. Make sure to remove all the waste accumulated near the floor.

Difference Between Cotton Wool Disease and Fungus

Cotton wool disease and fungus are quite similar to look at. Fungal diseases often appear as fin or body fungus.

Both diseases will cause woolly growth on the skin and other parts of the body. If left untreated, the growth can prove deadly for the fish.

The surefire way to differentiate between the two is to check under a microscope. In the initial stages, both these diseases can be easily identified.

Cotton Wool Disease will have bacterial growth. On the other hand, the fungus has thin, stringy filaments that resemble threads.

Another difference between the two is that fungus will always look like a cottony growth on the skin.

Cotton Wool Disease starts off from an area covered with mucus. The cottony growth comes much later.

The last difference is that fungal infections will always have white color. However, Cotton Wool Disease can have different colors like yellow, brown, gray, and white.

Frequently Asked Questions (FAQs)

What are the symptoms of Cotton Wool Disease?

The symptoms of Cotton Wool Disease include bacterial growth in the fish. This growth is usually white, but it can be yellow, gray, or even brown.

Your fish may secrete a lot of mucus from their skin. In general, fish suffering from Cotton Wool Disease do not show any willingness to swim around or eat their food.

What Causes the Cotton Wool Disease in Aquarium Fish?

It is caused by a freshwater bacteria strain (Flavobacterium columnare) which is quite common.

This strain is, in itself, harmless, but it can develop to become cotton wool disease.

Can Cotton Wool Disease infect humans?

It is unlikely that cotton wool disease would infect humans, given that the bacteria can live in freshwater only.

However, you should always wash your hands whenever you are working around your fish tank, just in case.

How long does the treatment take to be effective?

When properly administered, it usually takes four to six weeks to heal cotton wool disease.

Can Cotton Wool Disease be cured?

Provided that you start the treatment procedures right in the beginning and you take appropriate steps, the cotton wool disease can indeed be cured before it does any lasting damage.

However, once it has matured and the affected area has started to decay, it becomes progressively harder to treat it. Hence the emphasis is on treating the disease in its initial stages.


Cotton Wool Disease is a common problem among freshwater fish. In its early stages, it’s not a crisis for an aquarist.

Yet, aquarists do need to move fast to make sure the disease doesn’t spread.

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