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

By Nadine Oraby | 2020 Update

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

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

For aquarist, this can be an extremely worrisome and confusing situation.

But what actually is this growth on your neon tetra’s mouth? A growth on the mouth of a neon tetra is usually caused by two things; a fungal infection or neon tetra’s disease. Although a fungal infection can be treated with medication, neon tetra’s 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 most probably caused by either a fungal infection or neon tetra’s disease.

A good way you can 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 colour.

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

Another important thing to note is that both of these diseases can also affect other species of fish too. This includes other sub-species of the tetra family, as well as members of the cichlids and cyprinids family.

How to deal with a fungal infection

This disease is also more commonly known as mouth rot disease, cotton wool disease or cottonmouth. It’s also pretty common and can usually be cured if recognized in its earlier stages and treated accordingly.

The cause of this disease is actually a type of bacteria called Flexibacter Columnaris and are found in most aquariums. Although fish can fight off this disease when it’s present in a limited amount, sometimes it can develop and cause more severe symptoms that might become fatal to your fish if left untreated.

How fish contract a fungal infection

Probably the most active and most used part of a fish’s body is its mouth. From feeding to building nests to grabbing things, they use their mouth for everything which can make it especially vulnerable to bruising and diseases.

When your fish’s mouth gets wounded, it can allow bacteria to enter through that wound and cause an infection. Here, the bacteria multiply and starts gradually growing into a kind of fuzzy cyst.

The bacteria can also enter through the fins and gills of your fish if they get injured, resulting in a similar fuzzy growth on those body parts.

Symptoms of a fungal infection

In the initial stages, the mouth of your fish will start losing colour and will adopt a white-greyish colour. You might be able to recognize this if you closely inspect the mouth of your fish.

Soon after, this change in colour can turn a small, cotton-like growth. Along with this, you might also notice that your fish is losing their colour and will start becoming less active.

In later stages, they will also start distancing themselves from their school and will swim alone. If the growth gets even bigger, they might have trouble eating or might not eat at all. This is the stage where treating the disease can become difficult as your fish might die from hunger if they’re not able to eat.

It’s best to keep a close look at your fish and start treatment as soon as possible. Even if you think that their lips are losing colour, it’s best to keep an even closer watch on them for any signs of growth so the likelihood of your fish recovering can increase.

Treating a fungal infection

The very first thing you should do is to remove all the fish that you suspect have developed the infection from the main aquarium, and place them into a separate bowl. This prevents them from infecting any other fish and reduces the stress of those that are infected, so that they may recover faster.

Next, you should start medicating them by using an anti-bacterial or anti-fungal medication. You can find multiple fish medicines in fish stores that are aimed towards treating infections in fish.

Follow the instructions on the medicine you’re using and see for any progress. If you don’t see any improvement, try any other medicine to see if they work.

We also recommend you do water changes if you don’t see any progress after a week or so. Anywhere around 30-50% would be enough to help your fish start healing.

If you see that the fuzzy ball is slowly disappearing and your fish is becoming active again, continue the medication until symptoms until they are completely gone. After it’s been a week since the symptoms have disappeared, you can place your fish back into the main aquarium.

Causes of fungal infection

It’s very important that you know what actually causes fungal infection in fish. Like mentioned earlier, the cause of this infection is a bacterium called Flexibacter Columnaris.

These bacteria feed on the filth and waste material in your tank. This includes any excess food that’s left in the tank, fish faeces, and even a dead fish if they’re lying in the aquarium. Once they’ve started feeding on these things, they can multiply very quickly and can grow on many surfaces around the tank.

Therefore, probably the only way to keep these bacteria at bay is to keep your tank clean. Make sure you do regular water changes and remove any excess food from the tank once your fish are done eating. Every once in a while, lean the tank and all the decorations and ensure that the filter is running smoothly as well. All of these precautionary methods can help prevent this bacterium from potentially harming your fish.

How to deal with neon tetra disease

Another reason for that growth on your tetra’s mouth might be neon tetra disease. This disease is caused by a group of parasites called Microsporadia, a kind of fungi that can transfer through food or another infected fish.

Neon tetra disease might affect more than just tetras, fish such as angelfish, rasboras, danios and other of similar species. The bad news is that this disease is currently untreatable, and the best option would be to euthanize your fish to prevent the disease from infecting others.

How do fish contract neon tetra disease

The main way through which neon tetra disease is through infected food or fish. A parasite called Pleistophora hyphessobryconis can infect food, particularly live food, which you put into your aquarium. When a fish eats this food, the parasite enters their body where they start messing things up.

Once this parasite is inside their body, it starts to internally consume the fish. First, they manage their way through the intestines and then towards the skin where they form cysts. These can appear anywhere on their body, including their mouth.

Another way your fish can contract this disease is by consuming another infected fish. If a fish with neon tetra disease dies, and another fish starts eating them, the parasite can easily transmit to their body as well and cause the same symptoms. Because this disease is so contagious, it’s important to follow the right steps to prevent it from spreading all over your aquarium.

Symptoms of neon tetra disease

At the very beginning of the disease, you’ll initially notice a change in the behaviour of your fish. They’ll start swimming on their own and will seem quite restless. With time, their behaviour will become even more unpredictable, indicating an underlying issue.

This is followed by fading of colour of your fish. Their body will start becoming pale and white in colour, particularly their spine region. You will also notice your fish having trouble swimming straight or properly, as well as bumps and bulges on their mouth and body, which means the disease has progressed.

If there is growth on their mouth, the growth will be more even and smooth, instead of fuzzy. This is a very clear indicator between fungal infection and neon tetra disease. You might also notice that your fish is having trouble eating or isn’t eating at all.

Treatment of neon tetra disease

Unfortunately, neon tetra disease cannot be cured by medicine or any other method as of yet. You must take precautionary steps and separate them from the other fish as soon as you can.

Once they’re separated, you can continue to keep them in that condition. But if they’re struggling too much or are not eating, your best option would be to humanely euthanize them to prevent from further suffering.

Causes of neon tetra disease

As mentioned earlier, neon tetra disease is transmitted through food or infected fish. Whenever you buy live food, make sure that to obtain them from a reliable source like a pet store. If you feed them live food without ensuring that it’s clean and cooked, you might transfer the parasite to your fish.

Similarly, it’s crucial that you spot this disease as soon as possible. Even if you think there’s a slight chance that your fish has it, quarantine them as soon as you can.

If you leave them in the aquarium, they might get into a fight, and another fish might accidentally consume their tissue and get infected as well. Or maybe, they might die in a hidden corner, and by the time you find and remove them, another fish might have already become infected by consuming them.

You have to prioritize the life of your other fish and remove any fish you might suspect may have developed neon tetra disease.

Neon Tetra | Beginner Guide


Now that you know that there are only two main causes of growth on a tetra’s mouth, you can follow some steps to treat or euthanize them depending on what they’ve contracted. In spite of all this, it’s very important that you always maintain hygiene in your tank as well as the food you feed your fish to prevent them from all kinds of diseases and sicknesses. Similarly, it’s always the best option to separate any sick fish and place them in a separate container to prevent the outbreak of diseases.

Read more:

Leave a Comment