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Growth on Neon Tetra’s Mouth: Possible Causes & Treatments

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Many times neon tetras present signs that seem odd. For example, quite often, I found growths on my tetras’ mouth. Over the years, I researched that phenomenon and learned its possible causes and treatments. Now, I am willing to share my experience.

The growth on a neon tetra’s mouth is typically Columnaris, a disease caused by the Flavobacterium columnare. This condition is associated with a fuzzy, cotton-like growth on the upper lip of the tetra’s mouth. However, lip lumps may also be due to parasitic infections, tumors, and Lymphocystis.

As we move forward, I will teach you how to treat Columnaris and present a few other reasons that might have caused a lump to appear on your tetra’s mouth. For each one, I will explain what steps you should take to deal with the issue. 

What is the Growth on my Neon Tetra’s Mouth?

1. Columnaris (Cotton Wool Disease)

Columnaris is also called cotton wool disease. Some people confuse it for a fungal infection,[1] but it is actually caused by a bacterium (flavobacterium columnare) that is shaped like a column, hence the name:

You don’t have to worry about introducing these bacteria to your tank. They are already present.[2] You will find them in most, if not in all aquariums, which tells you that they are mostly harmless. In fact, they are helpful.

Your aquarium needs them to break organic material down. They only become a problem when they invade your fish’s body, which can happen when a neon tetra is injured in a fight. They will infiltrate the creature via open wounds (or gills).

Suppose the neon tetra’s immunity is weak due to stress, a bad diet, or poor water conditions. In that case, the bacteria will take root, developing and eventually killing the fish if the aquarist fails to take the necessary steps.

If your tetra suffers from columnaris, you may notice additional symptoms. These include lesions, spots, and patches on the body, especially the head, gills, and fins.[3] In some cases, the fish presents labored breathing and lethargy.

It is also worth mentioning that this condition is highly contagious. Once you suspect that your neon tetra caught Columnaris, I encourage you to quarantine the sick fish. Otherwise, the disease may spread to other fish in your tank. Yet, the disease doesn’t infect humans.

2. Neon Tetra Disease

Neon Tetra Disease is caused by the Pleistophora Hyphessobryconis parasite, which you can introduce by adding infected fish to your tank. It can also hitch a ride on the food, infecting your fish once they eat it.

It eventually makes its way to the skin, causing cysts to manifest. They are not restricted to the mouth. These cysts can appear in a variety of places across the neon tetra’s body. An infected fish will also begin to fade in color. It will also become restless.

The creature’s body will become lumpy, and it will also struggle to swim. Depending on the severity of the case, the neon tetra’s spine may curve. Neon tetra disease should concern you. It spreads pretty quickly.

3. Lymphocystis

This ailment is caused by a virus from the Lymphocystivirus genus (Iridoviridae family).[?] Your neon tetra can get it from an infected fish. The virus can also survive in water for a week. Some fish carry it without showing symptoms.

Infected creatures can develop tiny pin-pricks that eventually clump together to produce growths that are reminiscent of cauliflower. Besides the mouth, the clumps can appear on the gills, fins, and skin. If you think your neon tetras have Lymphocystis, consult a vet. They will test a scraping of the infected fish’s skin.

4. Your Tetra Developed a Tumor

It may surprise you to learn that tumors can manifest in fish. In some cases, you will see the masses on the lips. If your neon tetra’s situation gets worse, the growths will make it nearly impossible for the fish to eat or breathe.

5. It’s Not a Growth

First of all, you should take a moment to ensure that your fish has actual growths on or around the mouth. Sometimes, water plays tricks on your eyes, and what might look like growths from a distance could be something else entirely once you take a closer look, including:

  • Scars – Fish can develop scars for any number of reasons. They may fight with their neighbors. They could come under attack from larger, more violent fish. They may also collide with the objects in the tank. If the neon tetra in question develops bruises and wounds around the mouth, an aquarist may confuse the resulting scarring with growths.
  • Ich – Ich is an illness that causes white spots to appear all over the neon tetra’s body. In the rare situation where the spots are concentrated around the mouth, and the fish’s color is such that the rest of its body’s spots are not as vivid, you might confuse the spots around the mouth for growths.[4]

Of course, in such cases, once you begin your investigation, it won’t take you long to identify the rest of the spots on the fish’s body, in which case, you can conclude that the neon tetra has ich.

  • Ulcers – Like the scarring, ulcers caused by everything from bacterial erosion to chemicals may trick you into thinking that your neon tetra has growths around its mouth.

You should investigate your neon tetras carefully before taking action. First of all, this will prevent you from confusing benign elements like scars for growths. Secondly, growths are not all the same. 

Their appearance will make it so much easier for you to identify their cause. For instance, growths related to neon tetra disease are solid, whereas those caused by fungal infections are fuzzy.

Also Read: 17 Neon Tetra Diseases & Their Treatments

How To Treat a Neon Tetra with a Growth on Its Mouth?

As was noted before, growths on the mouth are a sign that things have gone wrong. The ailments that cause growths to appear can kill your fish if you fail to act quickly. Some practical steps that you can take include:

1. Columnaris Treatments

If your neon tetras have cotton wool, cottonmouth, or saddleback disease, you can attempt several treatments, including:

  • Antibiotics – Apply products like Furan, Terramycin, and copper sulfate.[5] Depending on the product, you can either treat the neon tetra through its food or a bath. On that matter, I suggest consulting an aquatic vet.
  • Salt – Salt alone is unlikely to cure your fish. However, it can reduce osmotic stress. That is where I usually recommend getting the API AQUARIUM SALT (link to Amazon). All you have to do is to add one tablespoon for every five gallons of water.
  • Temperature – Because a high temperature can accelerate the infection, you are encouraged to lower the temperature while treating the infected fish.
  • Isolation – As was mentioned earlier, the condition is contagious and could pass to other fish in your tank. Hence, I suggest quarantining the sick fish. You should also follow your fish to see if they develop signs of infection. 

I also recommend maintaining the temperature stable. Consistent fluctuations will dramatically stress your fish and lower the chances for a full recovery. On that matter, I highly suggest considering the Cobalt Aquatics Neo-Therm Pro Aquarium Heater (link to Amazon).

You can prevent future bouts of columnaris by cycling your tank, changing the water regularly, and maintaining the appropriate parameters. It would help if you also eliminate any factors that cause stress in the neon tetras. That includes violent neighbors.

If you feel confused, here is a helpful Youtube video that talks about this condition and its treatments:

2. Dealing With Neon Tetra Disease

If your neon tetras have neon tetra disease, you should immediately quarantine them. The illness has no cure at the moment.[6] Some infected fish can live for quite a while. You are expected to euthanize them all the same because they cannot be cured. However, if you want your fish to die naturally, keep it in quarantine. Otherwise, it will infect the other fish. 

Sick fish are just as dangerous when they die. If you permit them to remain in the community tank, once they die, their neighbors are just as likely to eat their remains before you can remove them. This will enable neon tetra disease to spread. This is why sick fish must remain in quarantine regardless of how long they have left to live.

Also Read: Neon Tetra Disease

3. The Approach to Fish Tumors

A vet will remove and then test your fish’s mass to determine an effective treatment. Vets can also perform surgery on the mouth. Though, the procedure is dangerous for the neon tetras. 

4. Treating Lymphocystis

Unfortunately, there is no cure for this disease.[7] Your only option is to improve the neon tetra’s conditions in the tank by eliminating all sources of stress. You can also use bacteria treatments (such as antibiotic baths) to combat the virus. 

Even if the infected neon tetra survives the virus, the growths can attract fatal bacterial infections, so bacteria treatments are encouraged. If you are fortunate, the effects of the virus will reverse over the weeks and months. 

Depending on the severity of your case, you should consider consulting a vet about the possibility of surgically removing the sections affected by the disease. None of these solutions can guarantee a positive outcome.

5. Adjusting the Tank for Neon Tetras

If your tetra developed a mass on its mouth and you are not sure what to do, start by improving your fish tank’s water conditions. Neon tetras are more likely to fall prey to infections if you force them to live in poorly maintained tanks. 

They will also recover at a faster rate if they have a stress-free, conducive environment: 

  • Getting the Right Tank Size – These creatures should be kept in at least 10 gallons of water. Though, to be on the safe side, you should aim for 20 gallons. A crowded tank induces stress which, in turn, makes the fish susceptible to diseases. 

Crowded tanks are also harder to maintain because waste and toxins like ammonia accumulate so quickly.

  • Setting the Proper Water Parameters – For neon tetras, it would be best to maintain a temperature of 70 to 81 degrees F. The pH should stay below 7. Do not allow the hardness to go beyond 10 dGH.[8]

To measure the pH, ammonia, nitrates, and nitrites, I highly recommend getting the API Aquarium Test Kit (link to Amazon). That bundle is highly affordable and easy to use. It also offers over 800 measures, making it significantly cost-effective.

Regarding the water hardness, I would pick the Premium Water Hardness Test Kit (link to Amazon). If you find the water too hard, use rainwater instead of tap water. As long as your water hardness is between 2 to 10 dGH, you are okay.

  • Taking Care of the Environment – Give the fish places to hide. Add vegetation and decorations like driftwood that create hiding spots they can use to secure their privacy. That will mitigate stress and strengthen the fish’s ability to overcome diseases.
  • Adequate Filtration – If you have neon tetras, you can get away with a sponge filter. The fish don’t produce that much waste. But you still need a filter. Don’t keep them in an aquarium that doesn’t have one.
  • Proper Water Changes – Change the water every week. This will keep toxins at bay while also maintaining the hygiene of the tank.

Be sure to cycle the tank before you add the neon tetras.[9] A tank that isn’t fully cycled will kill your neon tetras. Some aquarists will discourage you from keeping neon tetras in tanks that are younger than six months.


If your tetra developed a growth on its mouth, you should first examine its characteristics. A fuzzy, cotton-like lump is usually due to Columnaris, a disease caused by a bacterial infection. Treating it involves isolating the fish and using different kinds of antibiotics.

However, solid lumps could be tumors. In this case, there is nothing you can do. Luckily, the condition isn’t contagious. Regardless of the situation, it is better to adjust the tank to the tetra’s requirements. That will help the fish to deal with its ailment.