Growth on Neon Tetra’s mouth! – What to do?

So, one day, you’re just casually watching your tetras.

However, you notice that one of them has a growth on their mouth.

For aquarists, this can be a worrisome and confusing situation.

But what is this growth on your neon tetra’s mouth? A growth on the mouth of a neon tetra is usually one of two things, either a fungal infection or neon tetra disease. The fungal infection is treatable with medication, but neon tetra disease cannot.

However, there are still steps you need to take to protect the other fish in your aquarium. Read on below to know more about both these diseases:

What is the growth on my neon tetra’s mouth?

As mentioned earlier, a growth on a neon tetra’s mouth is likely either a fungal infection or neon tetra disease.

A good way to recognize which one your fish is suffering from is by taking a closer look at the growth itself. In the case of a fungal infection, the growth will have a fuzzy or cotton-like appearance, and your fish will likely appear a bit dull in color.

However, in the case of neon tetra disease, the growth will have a more solid appearance and look like a bulge on their mouth rather than a fuzzy ball. Apart from the mouth, these cysts may appear on other parts of their body too.

Another important thing to note is that both of these diseases can affect other fish species, including other sub-species of the tetra family and members of the cichlids and cyprinids family.

What is a fungal infection?

Fungal infection on a neon tetra’s mouth is also known as ‘Cotton wool disease.’ It occurs when Saprolegnia and Achyla fungi take over the immune system. Fish use their mouth to eat, making them vulnerable to fungal spores found in fish tanks. These spores can colonize and create problems for stressed and injured fish.

Most aquarium owners can identify external fungal infections like ‘cotton mouth’ on fish. It has a characteristic white fluffy appearance. As the fungal condition worsens, it may take on a grey or even red appearance.  

Symptoms of a fungal infection

The main symptoms of a fungal infection include:

  • Neon tetras will lose the color of their mouth. The natural blue-red color will change into a white or grey color
  • Fuzzy cotton-like growth on the mouth
  • Neon tetras may become less active than before
  • Neon tetras may separate themselves from other fish. This stage can be dangerous if your tetras stop eating because of the lumpy growth  
  • Experts recommend keeping a close eye on your neon tetras. If you observe any abnormal growth or behavioral changes, you should start the treatment immediately.

Experts recommend keeping a close eye on your neon tetras. If you observe any abnormal growth or behavioral changes, you should start the treatment immediately.

Treating a fungal infection

You can treat a fungal infection at the initial stage, so you must spot the growth early. As the lump size increases, it becomes harder to treat.

First things First:

Separate the infected tetras from other fish in the aquarium. Fungal infections are contagious and may affect others if kept together.

The next step is to start medicating. You can get multiple anti-fungal or anti-bacterial medicines from fish stores. If you can visibly notice signs of fungus, we recommend using Maracyn. Alternatively, you can visit your nearest pharmacy, and the chemist will recommend the best medicine for your neon tetras.

Before you begin treatment, clean up the aquarium and remove any chemical filtration. Follow the instructions on the packaging by mixing one packet in 10 gallons of water and repeat every 24 hours for five days. Make sure to complete the 5-day treatment of anti-fungal medication for the best results.

After a week of full recovery, you can keep your neon tetras back with other fish.

See also  How do Platy Fish Mate?

Neon Tetra Disease

Neon tetra disease refers to an infection caused by a Microsporidian parasite. It is more common than many aquarium enthusiasts realize and affects fish species beyond neon tetras. This disease is degenerative, meaning it starts slow but then progresses quickly to become quite severe.

Symptoms of Neon Tetra Disease

In neon tetra disease, you’re likely to observe these symptoms:

  • Restlessness
  • Neon tetras begin to lose coloration, often around their mouth
  • As cysts develop, the body may become lumpy
  • Neon tetra has difficulty swimming
  • In severe cases, the fish spine may become curved
  • Secondary infections such as bloating and fin rot

During the initial infection stages, the only symptom may be restlessness at night. Often the first thing you will notice is that the affected neon tetras isolate themselves—a clear sign that something is wrong. Eventually, swimming becomes erratic, and the growth is apparent.

As the infection progresses, affected muscle tissue starts to turn white, and the pale coloration expands.


There is no known treatment for neon tetra disease. If you are neon tetras struggle with this disease, experts recommend humanely euthanizing them. To ensure all your fish are not lost, remove infected fish from the tank. Some species may live for quite some time with the infection, but they should not be in a community tank. Separate them quickly to avoid spreading the disease.

What is a fish tumor?

Research shows that fish can have tumors as well. Most tumors appear as lumps or bumps on the fish’s skin. If you keep the infected neon tetras in the fish tank for a long time, it spreads to other fish.


If your fish suffers from a tumor, you will notice a pea-sized growth or oval papillary masses on the mouth. If the growth increases, neon tetras experience difficulty in breathing and eating.

Treatment for tumor

The diagnosis of the tumor depends on the biopsy results of the neon tetras. Generally, computerized ultrasonography and tomography can help check the tumor of neon tetras.

Excisional biopsy is a crucial part of the treatment of tumors in neon tetras. In this treatment, doctors remove the infected mass. Another treatment option is mouth surgery, which can be dangerous. There is a high risk of dying for neon tetra in this surgery.

How to Prevent Abnormal Growth on Neon Tetra

The best prevention is to maintain high water quality and to avoid purchasing sick fish. It’s vital to remove sick fish from your aquarium to curb disease transmission.

Always buy fish from a well-reputed supplier. If you’re buying online, be sure to check out reviews. Buying fish from a local supplier gives you a chance to observe the fish for abnormalities. Do not purchase any sick, dying, or dead fish.  

Once you’ve selected your favorite pets, quarantine new fish for at least two weeks before adding them to an existing aquarium to give your new pets a chance to adjust to a new environment, you can also observe their behavior and appearance. If you see any sickness signs, contact your supplier, and avoid letting sick fish interact with others.

It’s easy to end up with contaminants in fish supplies. So always select fish foods from a reputable and respected source.

Keep the environment healthy

When it comes to preventing fish disease, your aquarium’s cleanliness is crucial because it prevents deadly pathogens from thriving.

Experts recommend that you clean your aquarium every week. It ensures that the tank maintains a balanced and stable environment for the fish. Here are some cleaning measures you should undertake:

– Water Changes

Water changes are the most critical parts of routine aquarium maintenance. It’s best to change 10-15% of your tank water every alternate week. Keep in mind that the tap water contains chloramine (chlorine and ammonia) or chlorine.

See also  How to Tell If a Neon Tetra Is Going To Lay Eggs

While chlorine airs out in a bucket (aerated), chloramine doesn’t. Therefore, you should neutralize the chloramine with a water neutralizer.

Use a siphon to extract the tank water so that you get rid of uneaten fish food, fish excrement, and other harmful wastes.

– Water Testing

Regular water testing lets you check the tank’s water quality.

Testing aquarium waters can help identify and prevent a myriad of problems. It also depends on the level of imbalance in the water. If you notice unclean and contaminated water, you must clean your aquarium and replace the water thoroughly.

Here are some guidelines for testing water:

Nitrates– Nitrates in reef and saltwater aquariums should be 5 ppm (or lower) and 10 ppm in freshwater tanks.

Nitrites- Your aquariums should have undetectable nitrates. If there are nitrates, then you’ll also notice high ammonia levels.  

pH– Always maintain the water pH levels at a range of 6.5 to 7.5, which is the most suitable range for all fish species.

KH (Carbonate Hardness)– KH determines the pH levels in your fish tank. For example, a carbonate hardness drop to around 4.2 dH (degree hardness) indicates that the pH of your aquarium is unstable.

– Filter Maintenance

It’s important to service your aquarium’s filter to avoid poisoning your fish’s home.

Filters are receptacles for waste and need replacing every month. Servicing a fish tank filter is a simple process. It involves taking the filter out and replacing the dirty filter inserts. You may also want to conduct a complete filter rinse every four weeks.

Neon Tetra | Beginner Guide

Frequently Asked Questions (FAQ)

How long can my fish live with neon tetra disease?

Healthy Neon tetras normally can live for ten years in wild nature and five years in a fish tank. You should avoid bringing any drastic changes to an aquarium because it can traumatize the neon teras. Neon Tetra Disease (NTD) is an incurable disease, but you can prevent it.

Neons with NTD can stay alive for a month, depending on how fast the parasite is growing. So, you must be careful when you buy live fish food for them as they are omnivorous.

Do I need to isolate infected neon tetra fish?

If you notice any growth in their mouth, separating a neon tetra from other fish is crucial. It’s a contagious disease and can infect other fish. If you keep them in the same fish tank, other fish might consume the infected tissue and die. Separating infected neon tetras can also reduce stress and help them recover fast.

Can other fish species get neon tetra disease?

You can observe this disease with other fish species like Angelfish, Rasboras, and Barbs. If you find any fish with this disease, just isolate them and start the medication.

Can I use aquarium salt?

If you add the right proportion of aquarium salt, neon tetras can handle it better than other fish. The tolerance level of other fish with aquarium salt is much lower. But if you add excessive salt, neon tetras may not adapt.

Aquarium salt is ideal for the treatment of common fish diseases like itching. Aquarium salt isn’t suitable for tropical fish, so you’ll have to be careful while using it. Read the instructions written on the aquarium salt to use the right proportion.

Is cottonmouth in neon tetra contagious?

Flexibacter Columnaris is the main cause of fungal infection or cottonmouth. It is a contagious disease and transmits quickly. If a healthy fish consumes the infected tissue, then the healthy fish might also catch this infection. So, isolate the sick fish to prevent infection spread.

Photo of author

Nadine Oraby

My name is Nadine; I am a passionate writer and a pet lover. People usually call me by the nickname “Joy” because they think that I am a positive and joyful person who is a child at heart. My love for animals triggered me to create this blog. Articles are written by vets, pet experts, and me. Thanks for visiting. Your friend, Nadine!

Leave a Comment