Freshwater Fish with Cotton Wool Disease

Guppy with fuzzy growth on dorsal fin

Cotton wool disease is also known as cotton mouth disease, columnaris disease, saddleback necrosis, and black patch necrosis ( Flavobacterium columnare ). Because of its light hue and elevated appearance, this bacterium is sometimes mistaken for a fungus. It may infect most freshwater fish species, however it typically occurs as a result of another major stressor. Some strains are more infectious and lethal than others.

What is Cotton Wool Disease?

The bacteria Flavobacterium columnare causes cotton wool disease. Despite its mushroom-like look, it is not a fungus. It may infect the skin and gills and is vital to the commercial aquaculture industry. It's more uncommon among pet fish. It's mostly an opportunistic infection that preys on stressed fish with weakened immune systems. Within the species, there are several strains, some of which are substantially more lethal and spread more quickly.

Symptoms of Cotton Wool Disease in Freshwater Fish

Cotton wool sickness symptoms are typically evident since they are visible, however they might be mistaken for other fish illnesses. The most noticeable indication of the condition will be the appearance of fluffy skin patches. The following is a comprehensive list of symptoms:


  • Skin patches
  • Pale gills
  • Lethargy
  • Difficulty swimming
  • Loss of appetite

Skin Patches

A pale, raised patch on your fish's skin is the most typical clinical indication of cotton wool sickness. It can appear on the face, lips, or fins, among other places on the body. These lesions have a fluffy look that resembles a fungal growth.

Pale Gills

In some instances, these skin patches can spread to the gills. The gill tissue may then appear necrotic or pale when examined.


As the bacteria replicates and spreads throughout your fish's system, your pet will feel stressed and lethargic.

Difficulty Swimming

The spreading illness and patches can cause your fish to lose its equilibrium and cause it to have difficulty swimming.

Loss of Appetite

A sick fish often experiences a loss of appetite.

Causes of Cotton Wool Disease

Columnaris bacteria are commensal bacteria that live alongside healthy fish. If the fish is agitated or if the bacterium strain is extremely unpleasant, clinical indications appear. When diseased fish are brought to a tank without adequate quarantine, the bacteria can easily invade a healthy system. It's crucial to monitor your since columnaris bacteria like warmer water (about 80 degrees Fahrenheit), which means they'll cause issues more quickly than if they're in colder water.

Diagnosing Cotton Wool Disease in Freshwater Fish

It's crucial to distinguish between columnaris species and fungal infections. A tiny biopsy will be taken from a sedated fish and examined under a microscope by your veterinarian. If cotton wool disease is present, bacterium rods will form small haystacks, however fungus will not, and individual hyphae (branching filaments that are part of a fungus) may be distinguishable.

Additional testing, such as sending a swab sample to a lab to identify the strain and determine the most effective medications, may be necessary. With antibiotic therapy, never "guess." You risk destroying your biologic filtering system by utilizing the wrong product or dose, which can breed more resistant germs in your system.


All symptomatic fish should be placed in a hospital tank. Your veterinarian will either give you an antibiotic to put in the water or an injectable medication. Antibiotics that are injected do not harm your and give a more powerful therapy alternative. Never try to inject your fish on your own.

If no sample is received for culture and sensitivity testing, your fish may require other antibiotics. To determine which antibiotic is appropriate for your condition, you'll need a comprehensive history of your fish, including what therapies have already been tried.

Prognosis for Fish With Cotton Wool Disease

Fish that are severely unwell, such as those that have lost more than half of their gill tissue, may need to be euthanized. Fish have a difficult time recovering from such a serious illness. If your fish's gills have been affected, using an to provide additional oxygen support is vital.

Because it is a secondary invader, all possible stresses in your fish system should be evaluated. This involves checking your water chemistry, analyzing your fish's food, comparing fish compatibility, and looking over any new fish or d├ęcor you've added. If the main problem is not addressed, your fish will become unwell again after therapy is completed.

How to Prevent Cotton Wool Disease

As with most diseases, cotton wool disease can be prevented with proper quarantine. Take these steps to keep cotton wool disease out of your tank.

  • Quarantine: Set up a completely separate system with separate filtration and other equipment for four to six weeks. Two weeks is not adequate time and maintaining the correct temperature in a tank is very important.
  • Observe: Watch your fish closely during this time and note changes in their appearance and behavior. If you notice something wrong, it is critical to get it diagnosed fast and correctly to make sure it doesn't become a larger problem. By keeping your fish in isolation, you will protect your main system from infection.
  • Eliminate stress: It is also critical to keep your fish healthy in a low-stress environment. Make sure all fish have plenty of , a , and .