Neon Tetra Disease, Ich, Cotton Mouth and other Illnesses (Treatment Guide)

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Neon Tetras get sick with many of the same conditions that other fish suffer from.

Some are contagious and some are life-threatening if not caught early, but there are also others that your fish can live with for the rest of its long and happy life – even if they are stuck swimming upside down!

Despite the name, Neon Tetra Disease is not exclusively found in Neon Tetras, but if you have a disease named after you it’s easy to assume that any illness in a Neon is NTD.

Read on to find out what to look for when diagnosing a sick fish and how to treat the illness once you know what it is.

What does Neon Tetra Disease look like?

Neon Tetra disease comes with symptoms such as lumps along the body and a curved spine, which result in a lack of swimming. The earliest signs are fading colors, and infected fish no longer swimming with the rest of the school, becoming restless and swimming in odd patterns.

When the fish’s color begins to fade it may only be in a specific area of its body rather than all over, so make sure you look closely before dismissing this symptom. The lumps you may be seeing are cysts which develop as the disease takes hold.

After the initial stage, where the fish swims erratically away from other members of the school, an infected fish finds it difficult to swim and settles in one area for longer periods. This is because the cysts start to deform the muscles, making it hard for the fish to swim. This is what causes the characteristic curved spine in advanced cases of NTD.

Credit: u/nathdcfc on Reddit

Eventually infection sets in, which causes fin rot and bloat and it becomes unlikely that the animal will survive.

How to treat Neon Tetra Disease

If your fish has Neon Tetra Disease, it must be removed from the tank immediately to avoid infecting other tankmates. As there is no known cure or treatment, most fish are euthanized either by being placed in a freezing bag of water, or by adding clove oil mixed with warm water to the container where the fish is being kept.

What does Ich look like on a Tetra?

Ich appears as white spots on the skin and/or fins. Ich is a parasite that burrows into the skin, so the fish’s immune system grows the white spots as a protective layer over the wound, so Ich spots look raised rather than a simple discoloration of the skin.

Cardinal Tetra with Ich. Credit: u/Reddogo on Reddit

How do you treat white spots/Ich on Neon Tetras?

Once you’ve tested the water in your tank for any unusual parameters, you’ll need to slowly increase the temperature of your aquarium to around 86°F (30°C) to speed up the life cycle of the parasite. Once done, you can choose from a number of Ich medications.

The reason for the temperature change is that the parasite is resistant to medication in egg form.

The single parasite removes itself from your fish and settles on the aquarium floor. Once there it creates a protective shell around itself and multiplies into thousands before breaking free and looking for another fish to infect.

At higher temperatures this process occurs more quickly and increases the likelihood of the parasite encountering the medicine, breaking the life cycle.

Take care not to raise the temperature too quickly though, as fish are sensitive to sudden changes.

FishLab have a great article which explains the behavior of the Ich parasite and the treatment of freshwater fish in more detail, which you can find here.

Note that if your fish is suffering from Neon Tetra Disease, white spots won’t be one of the symptoms. The cysts seen in fish with NTD are larger and grow under the surface, whereas like any other fish, a Neon Tetra with Ich will grow white spots externally.

Cotton Mouth (Columnaris)

Cotton Mouth is a common name for a fungal infection that commonly grows on the mouth, but can also appear in other areas. It is caused by a fungus that enters the body as the fish eats, which is why it affects the mouth.

One of my Neon Tetras during a Cotton Mouth infection

Because the cysts that appear in fish with Neon Tetra Disease also commonly grow on the mouth, NTD and Cotton Mouth are often confused, but these cysts are more ball-like whereas Cotton Mouth looks furry.

Also known as Cotton Wool Disease, the fungus can grow to the point where it prevents the fish from eating, causing them to starve to death.

What causes Cottonmouth in Neon Tetras?

Cottonmouth is caused by a bacteria called Flavobacterium Columnare which thrives in poor water conditions and at high temperatures. It enters through the gills or the mouth, or through any open wounds on the skin of the infected fish.

A tank with a high bioload and high temperature is the perfect breeding ground for Columnaris. So, if your fish prefer high temperatures, it’s vital that the tank isn’t over-stocked and that regular water changes are done to maintain good water quality.

How do you treat a Cottonmouth Neon Tetra?

Daily water changes and treatment with aquarium salt and antifungal medicines can get rid of Cotton Mouth. It’s also important to quarantine the fish in a hospital tank as fungal infections are very contagious.

Bear in mind that snails are particularly sensitive to aquarium salt and will likely die if they come into direct contact, so make sure you move the affected fish to a hospital before treating the condition, especially if you have snails.

Antifungal medications usually advise to remove any activated carbon from your filter media as the medication can cause a chemical reaction which is harmful to your fish.

Therefore it’s wise to give your fish a medicated bath and treat the tank itself with regular water changes and slowly lower the water temperature to make it harder for Columnaris to survive in your aquarium.

Why does my Neon Tetra have red gills?

In fish including Neon Tetra, red gills are a sign of irritation caused by toxins in the water. These toxins are many and varied, but ammonia is very common and causes burns on the sensitive gills of the fish, which appear red and inflamed.

Credit: u/ManCakes on Reddit

How to treat ammonia burns on fish

Regular water changes is the natural treatment for ammonia burns.

Change 50% of the tank water daily and treat with a dechlorinator such as Seachem Prime to ensure healthy water conditions for your fish.

Make sure you test your water regularly to check for ammonia and other harmful chemicals.

Read more:

Why are my Neon Tetra’s eyes bulging?

A Neon Tetra’s eyes bulging is usually caused by a bacterial infection called exophthalmia but commonly known as Pop Eye. It is caused either by poor water quality or by introducing an infected fish to the tank, which then passes on the condition.

How do you treat Popeye?

The first step to treating Popeye in fish is to improve the water conditions. No amount of chemical treatments will work for long in poor quality water, so perform daily water changes of around 50% and treat the new water with a dechlorinator.

Goldfish with Popeye. Credit: DarkFatigue on Reddit.

From there, medicines such as API Melafix will help your fish recover when included in the new water during your daily changes. Aquarium salt can be combined with the medicine to give your fish an even better chance of recovery.

What does Fin Rot look like on Neon Tetras?

Fin rot in fish such as Neon Tetras starts out as a simple fading of the color in the fin which is easily missed. However, as the condition worsens bits of the fin will fall away, leaving the fin and the surrounding tissue misshapen and inflamed.

As with any open wound, this can lead to secondary infection including Cotton Wool Disease (see Cottonmouth above) and it’s important that the disease is treated quickly to avoid further complications for your fish.

How do you treat Fin Rot in Neon Tetras?

To help your Neon Tetra recover from fin rot the water quality needs to be good, so perform daily 50% water changes and treat the new water with a dechlorinator, as tap water contains harmful chemicals. Then add antibiotics to the new water to help fight the disease.

Treatments such as API Fin & Body Cure will help your fish recover and promote the growth of good bacteria in the tank to keep your fish healthy in the long term.

You can help too by keeping up with regular water changes (10-20% of the water twice weekly is a good guide) and removing any uneaten food rather than letting it rot in the tank. Not over-feeding in the first place is the best way to prevent this, so only feed what your fish can eat in two minutes. Two smaller feeds per day is better than one big one!

Will Neon Tetra fins grow back?

The good news is that fish including Neon Tetras will regrow fins that have been damaged by nipping or fin rot. Only when there has been significant damage to the fins and the surrounding tissue could the damage be permanent, but this doesn’t mean your fish will die.

In good water conditions, fin regrowth should take around 1-4 weeks but if 8 weeks pass without recovery, consider another round of treatment. If you still don’t see the fins regrowing it probably means the damage was too great and the fins won’t grow back at all.

Bloated/swollen belly

There are a number of reasons your Neon Tetra might have a swollen belly, some of which are good news (if you want to breed your fish!) but others can be a very bad sign.

A bloated Neon Tetra. Credit: u/Katara_1 on Reddit

If your Neon is pregnant (or ‘gravid’), she can look huge as they carry hundreds of eggs at once. Of course, your fish could also simply have eaten too much, so keep your eyes open and watch to see if the bloating reduces when your fish haven’t recently been fed!

Read more: How Can You Tell If A Neon Tetra Is Pregnant?

There are things to be careful about if you notice your Neon Tetra is bloated. Check the food you’re using is appropriate for them and isn’t causing them to get constipated, which can happen when overfeeding with Bloodworm, for example.

More worryingly, bloat is a sign of advanced Neon Tetra Disease. Read the section above on NTD if you think this may be the cause of your Neon Tetra’s swollen belly, as this is extremely serious.

Swim Bladder Disease

The swim bladder is a pocket of gas that your fish uses to control its swimming and prevent it from floating to the surface or sinking to the bottom of the tank. When infected, the fish can swim nose down or even upside down, or float around the aquarium in unusual ways due to loss of control of their buoyancy.

How do you treat Swim Bladder in Neon Tetras?

Swim Bladder Disease is often caused by overfeeding in which case fasting is the best cure, or if poor water quality is suspected then daily water changes and parameter testing is the best approach. Antibiotic treatments in the new water will also help.

In an infected Neon Tetra, Swim Bladder Disease can be treated by not feeding the fish for 2-3 days. However there are many possible causes outside of overfeeding so perform daily 50% water changes for a week, and treat the new water with an antibiotic such as API Melafix and some aquarium salt.


It can be difficult to tell Dropsy apart from Swim Bladder Disease as obvious signs are similar for each illness. With Dropsy, the fish’s belly will be swollen (just like Swim Bladder) and you’ll notice your Neon Tetra becoming lethargic rather than being its usual active self.

A tell-tale sign of Dropsy is when the scales start to come loose and even fall away, while the fish’s eyes can become sunken or start to bulge. Finally, the fish will have difficulty swimming the right way up and may even start to swim upside down.

Goldfish with severe Dropsy. Credit: u/54B3R_ on Reddit

The scales extending out and swimming upside down are signs of advanced Dropsy and by this stage it’s often kinder to euthanize the fish as it’s usually too late to treat.

The good news is that Dropsy isn’t contagious so even if you lose one fish, you don’t need to worry about the others catching the disease, but they can develop the same symptoms if the underlying cause isn’t corrected.

Dropsy is caused by a bacteria which affects fish that are stressed by overcrowded tanks with poor water quality, so prevention is much easier than cure. Always make sure your tank is the right size to accommodate the number of fish you want to stock, and perform regular water changes to maintain good water quality.

Read more: Neon Tetra Tank Size: Everything You Need To Know

How do you treat Dropsy in Neon Tetras?

In an infected Neon Tetra, Dropsy can be treated by quarantining the fish and giving them a salt bath with antibiotics. Once the symptoms clear the Neon can be returned to the tank, but make sure the water quality is improved and the tank isn’t over-stocked.

API Melafix is a good choice for treating a wide variety of bacterial infections, including Dropsy.


How do you know if a Neon Tetra is dying?

If your Neon Tetra stops swimming with the rest of the school, stays at the surface or the bottom of the tank, or if it looks different (including discoloration, white spots, lumps or a mis-shapen spine), these can be signs of serious illnesses that can lead to the fish dying.

Why are my Neon Tetras dying?

Most illnesses that lead to Neon Tetras and other fish dying are the result of parasites or bacterial infections that thrive in poor water quality. The best way to prevent this is to avoid over-stocking the tank and keep water quality high with regular water changes.

What are the symptoms of Neon Tetra Disease?

Early symptoms of Neon Tetra Disease include fading colors in one area of the fish’s skin and unusual swimming patterns. As the disease develops you’ll notice a bloated belly and lumps under the skin (cysts) and as these cysts attack the muscles, a misshapen spine.

Will Ich go away on its own?

Some saltwater fish have natural defenses to help them fight off Ich, but in a home aquarium the only way to be sure is to treat the water with a specific antibiotic that breaks the parasite’s life cycle and kills it before it can infect other fish.

Is Cottonmouth in fish contagious?

Cottonmouth or Cotton Wool Disease is highly contagious and often leads to death due to the fish being unable to eat. For this reason, any fish contracting the disease should be quarantined and treated immediately.

How long does Popeye last in fish?

Popeye treatment can be fast acting if caught early but can also last for months, and while the swelling will eventually go away the disease can do permanent damage to the fish’s eye.

What are the symptoms of Fin Rot?

As the name suggests, Fin Rot symptoms include an initial discoloration of the fin and surrounding tissue, before parts of the fin actually start to fall away. There is also the danger of secondary infection as the condition causes open wounds.

How long does it take for Fin Rot to heal?

In severe cases the tissue damage caused by Fin Rot can be permanent but often the fins grow back within a week or two if treated early enough. If the symptoms persist for 8 weeks or more, the fish will need continued treatment and may not recover.

Does Swim Bladder go away?

Swim Bladder Disease is treatable and most of the time the fish recovers completely, but even if this isn’t the case for your fish they can still live a long and happy life, even though their swimming is affected – they may even swim upside down for life!

Should I euthanize my fish with Swim Bladder?

Swim Bladder Disease alone is not a good reason to euthanize an otherwise healthy fish, as they can live long and happy lives even if they no longer swim the right way up. Swim Bladder is not contagious so there is no danger to other fish in the tank.

Is Dropsy contagious?

Dropsy is not contagious but any fish in the same aquarium can also develop the illness if the underlying cause is not treated. Usually this is poor water quality caused by over-stocking the tank and inadequate water changes, which allows the bacteria to infect the stressed fish.

Can a fish survive Dropsy?

A fish can survive Dropsy if caught and treated early enough but if the scales start to extend and fall away, this is a sign that the disease is at an advanced stage and it’s often too late, so the kindest option is to euthanize the fish.

What causes Dropsy in Tetras?

Dropsy is caused when the bacteria commonly found in tank water infect fish whose immune system is weakened by stress. This stress is usually due to poor water quality, which is in turn caused by over-stocking or infrequent water changes.

Can overfeeding cause Dropsy?

Overfeeding can cause Dropsy in that leftover food starts to decompose in the tank, releasing ammonia into the water. Fish become stressed, which weakens their immune system and allows bacteria to infect them, leading to Dropsy.


Most of the illnesses described above can be avoided by maintaining good water quality, so always test your parameters regularly and perform the appropriate water changes at least once per week.

If you notice any of the symptoms described above, move the fish immediately to a properly-cycled hospital/quarantine tank and use the necessary medication to treat your fish.

Only return them to the main tank when you’re sure any water quality issues have been addressed and your fish’s symptoms are gone.

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