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11.12. Neon Disease

11.12. Neon Disease
11.12. Neon Disease

Neons get a lot of diseases.

Neons are not a Beginner Fish.

Neons need very clean, bacteria free water to do well.

Neon Tetra Disease
Neon Tetra Disease and Columnaris
A Predicate

Note that neons are just like any other fish in that they can die very rapidly under certain common conditions. Let us say you have a tank of neons which has been running a month. You do a 50% water change and do something else, like add bacteria in a bottle. The next day you have three dead neons. What happened?

Now all the well-intentioned commentators on social media will tell you that you had an ammonia spike overnight. And they will all be wrong. Ammonia only kills slowly, like in weeks. Others will say the bacteria in a bottle killed them. Bacteria in a bottle products are harmless. There are three distinct possibilities that will probably explain 90% of the deaths:

  • With a municipal water source a “chlorine pulse” is likely.
  • With well water high carbon dioxide or low oxygen levels in the water probably killed the fish.
  • It there was no water change (and sometimes with a water change) a fast moving bacterial infection (possibly columnaris) can rapidly kill neons 
Neon with ich
Neons with ich

Many water suppliers do something called a “chlorine pulse”. The water suppliers put up to 4 ppm chlorine or chloramine in the water. The normal dose of conditioner will neutralize 1 ppm. That leaves 3 ppm to kill the fish. 

Some well water will have large amounts of carbon dioxide in it. This can kill fish. Well water often has to be aerated for several hours before being added to the aquarium.

And finally because neons are what is called a “blackwater” fish, they can be killed very rapidly by several types of bacteria. This often occurs with no symptoms. Neons need very clear water that is typically only found in very mature aquariums (like a filter and substrate which have not been cleaned for at least two months).

Neon Tetra Disease
Neon Tetra Disease and Bacterial
Conditions for Neons

Experienced aquarium hobbyists who have aquariums with crystal clear water rarely see  many problems with neons. This fact is very revealing. Crystal clear water is generally over-filtered water with great aeration. Over-filtered water is bacteria free water. And neons do well in bacteria free water.

Neons come from blackwater. What people don’t realize about blackwater is that bacteria don’t thrive in blackwater. That is actually the reason the water is black! So neons come from water which has little bacteria in it. So a neon has a very poorly developed immune system. If a neon is put into water teeming with bacteria, such as most new aquariums have, the neons get all sorts of infections and die, often quite rapidly.

Neon Tetra Disease
Neon Tetra Disease with snout chondroma

Neons do best in aquariums which are over-filtered, with at least a canister filter, a sump and/or an under-gravel filter. Hang on back filters generally don’t give adequate biofiltration for neons and other sensitive fish like rummy noses, apistos and rams (note that a HOB can be considerably improved if the recommendations of Cory of Aquarium Co-op are followed).  Neons do much better in aquariums which have been established at least five months. Ideally the water should be crystal clear. Aeration should be excellent.

Preferably the aquarium has an in-line UV unit of at least 1 watts of power per ten gallons. All of this is to provide very bacteria free water. If neons are provided with very bacteria free water their natural immune systems can successfully fight off “neon tetra disease” (Pleistophora) and all the many other infections neons are prone to. This blackwater sensitivity is analyzed in more depth in this link:

17.3. Blackwater Fish

Neon Tetra with Columnaris fin rot
Neon Tetra with NTD, Columnaris fin rot and snout chondromas
Neon Diseases

Neons commonly get the following three diseases:

  • neon tetra disease, caused by a fungi
  • big white “warts” or snout chondromas on their mouths, sometimes with neon tetra disease and sometimes occurring alone. What causes these is uncertain but it is probably a manifestation of neon tetra disease.
  • saddleback and fin rot (bacterial diseases, generally columnaris), generally secondary to neon tetra disease but also alone.
Neon Tetras wiith all three diseases
Neon Tetras with all three diseases

Note that neons also commonly get the many other fish diseases such as ich and environmental mycobacterium (“Fish TB”).  They also frequently die for no apparent reason. These unexplainable deaths are typically internal bacteria infections.

To understand this in more depth read on. Note this is long, boring, and only suited for real nerds like the author. 

Neon Tetra Disease

Named after the fish that it was first identified in, neon tetra disease strikes members of the tetra family, most often neon tetras, but other types of fish are not immune, and the disease has been found in cardinal tetras, danios, zebrafish, angelfish, rasboras, and barbs. Here is a rummy nosed tetra with neon tetra disease:

Neon Tetra Disease
Neon Tetra Disease

Neon Tetra disease is caused by a parasitic organism known as Pleistophora Hyphessobryconis. This organism is a “Microsporidia”. Microsporidia are a group of spore-forming unicellular parasitic fungi, with a complicated life cycle. The life cycle concludes with the production of an infectious and resistant spore, which is the only stage of the parasite that can live outside of the host cell.

With most microsporidia, transmission occurs via the ingestion of the infective spore stage. Spores enter the fish through ingesting the bodies of infected dead fish.

Once inside a fish, the parasite will eat the fish from the inside out, starting with the digestive tract and stomach. Sporoplasms develop inside the intestines, burrow through the intestinal wall into the skeletal muscles, and there they produce cysts. The cysts damage the tissue; signs of tissue damage include paler color and weaker muscles.

Neon Tetra Disease
Snout chondroma in a neon

It’s important to know neon tetra disease is highly communicable and can easily spread through a tank quickly. The problem with neon tetra disease is that it doesn’t spread through the water. Rather it spreads through the digestive tract. When a neon with neon tetra disease dies the other fish eat it. When this happens the other fish are invaded by far too many of the Pleistophora for their immune systems to handle. So they get neon tetra disease. To save your fish, you will need to remove infected fish as early as possible.

This is a disease that is much like Fish TB. The organism infects the organs and causes wasting of the muscles along the spine and spinal curvature. Like fish TB it also causes lumps and cysts and weakens the fish for a whole host of other organisms to take hold. And like fish TB there is no effective medication. Also like fish TB neon tetra disease goes very slowly, with neons succumbing over a span of weeks or even months.

And, like TB, typically some of the neons will manage to fight it off. The cleaner the water the healthier the neons will be. And the healthier the neons are the better they can fight off the neon tetra disease.

It should be noted that neon tetra disease is very common with new hobbyists who put neons in new tanks with new filters. Experienced hobbyists typically do not get neon tetra disease when adding neons to their established fish tanks. The experienced hobbyists then come on social media and say neon tetra disease is actually a very uncommon disease. What they are missing is that it is uncommon in well filtered established aquariums but common in poorly filtered newly set up aquariums.

Neon Tetra Disease
Neon Tetra Disease
Symptoms of Neon Tetra Disease

The symptoms of neon tetra disease are:

  • Restlessness
  • Fish begins to lose coloration, with the white area of the belly expanding and/or white appearing along the spine
  • Sometimes the white areas become lumpy
  • The blue color line along the fish goes from a straight line to a “S” shape as the spine distorts due to the muscles wasting.
  • The tail takes a slight downward turn
  • Fish has difficulty swimming, swimming with its head down and with a lot of “twitching”.

During the initial stages, the only symptom may be restlessness, particularly at night. Often the first thing an owner will notice is that the affected fish no longer school with the others—a clear sign that something is wrong. Eventually, swimming becomes more erratic, the fish assumes an angle in the water and it becomes obvious that the fish is not well.

Neon Tetra Disease
Neon Tetra Disease

As the disease progresses, affected muscle tissue begins to turn white, generally starting within the belly and areas along the spine. As additional muscle tissue is affected, the pale coloration expands. Eventually the pale white coloration may cover most of the fish, including the blue line. Here are some neons with very advanced neon tetra disease.

Advanced neon tetra disease
Advanced neon tetra disease

Damage to the muscles can cause curvature or deformation of the spine, which may cause the fish to have difficulty in swimming. Any fat neon tetra (typically females) will have some “S” curve of the blue line but this becomes more pronounced with neon tetra disease. Thin neons which don’t have neon tetra disease shouldn’t have any curve to their spines. It is not unusual for the body of the fish with neon tetra disease to have a lumpy appearance as the cysts deform the muscles.

The only way to treat neon tetra disease is to cull the neons with the disease. Then give all the fish in the aquarium very over-filtered, bacteria-free, crystal-clear, well aerated water. By significantly reducing the bacteria count in the water the neons will have more immune system resources available to fight off the Pleistophora organism. Neons can and do fight off the Pleistophora organism if given water with low bacteria counts.

There are several ways to get bacteria free water:

  • If you have a hang-on-back filter improve the filter by  changing out the filter media to foam or pot scrubbers. For more details go to the link:

8.2. Hang on Back Filters. 

  • If you have a hang-on-back filter add another filter with good biofiltration capacity.  These include:
    • An under-gravel filter (the most cost effective option, and yes, they work great if they aren’t cleaned but once every few years!)
    • a canister with a good media like foam, pot scrubbers or static K1 media (a media like Matrix, Biohome, ceramic rings or lava rock typically won’t cut it)
    • a sump for larger aquariums
  • Only clean your filter media when the flow noticeably slows down. The brown gunk in the filter is good, beneficial bacteria. “Beneficial bacteria ain’t pretty!”
  • Add a UV unit
neon with neon tetra disease
neon with only very subtle neon tetra disease

The neon above has true “neon tetra disease” (Pleistophora). Note the “S” curved blue line, the slightly turned down tail and the extra white in the belly. Note this is difficult to distinguish from a normal female neon. The spinal curve is more pronounced and “hitched” with neon tetra disease.

This is a bunch of healthy fish for comparison. It is not easy to tell the difference! Do note how the fish are swimming. No fish is swimming in a head down position.

healthy neons
healthy neons
Mouth Warts

Neon tetras are very susceptible to a mouth growth which is of uncertain etiology and causes. When these tumors are sectioned, no Pleistophora (neon tetra disease pathogen) is found. But no other agents are found. But also neon tetra disease and these growths are often found together. These tumors are called “snout chondromas”. Chondromas are benign tumors that often do not have the initiating pathogen present. So Pleistophora (neon tetra disease pathogen) may well be the cause of these tumors but just no longer present in the tumors.

Any neon with a snout chondroma probably has neon tetra disease and should be removed from the aquarium and euthanized. Note that many well-intentioned commentators on social media say these growths are columnaris. They are not columnaris.

lip growths on neons
lip growths on neons

Note that while the first neon above obviously has neon tetra disease, the last three fish above do not APPEAR to have neon tetra disease. But in all probability they do have neon tetra disease and should be euthanized.

This mouth tumor is often accompanied by other pathogens:

Neon tetras with fin rot, neon tetra disease and mouth tumors.
Neon tetras with fin rot, neon tetra disease and mouth tumors.
Bacterial Infections

Neons  commonly get “columnaris” bacterial infections. While these infections can be a columnaris strain of bacteria they can also be about a dozen other bacterial species. Virtually all the species of bacteria infecting the exterior of a fish are gram negative.

Because this columnaris is typically but not always secondary to neon tetra disease any hobbyist that finds a bacterial infection with a neon faces a bit of a quandary. Does one euthanize the fish to prevent neon tetra disease from spreading or does one assume that the bacterial infection is not secondary to neon tetra disease?  I recommend euthanizing the fish.

If one doesn’t cull the fish and wants to treat the bacteria, the fish need to be treated with antibiotics IN THEIR FOOD. Add an antibiotic (Thomas Labs Fish Mox, Midland Vet Service Aqua-Mox, VetDepot Amoxicillin, SeaChem KanaPlex, Fishbiotic Ampicillin, Mardel Maracyn 2, Thomas Labs Fish Min, Thomas Labs Fish Doxy) added to ONLY the food. Treatment needs to be for at least two weeks. Treat all the fish in the aquarium as bacterial infections like columnaris will affect all fish species.

Note that most aquarium medications tell one to add the medication to the water. This is a profit driven scam. Freshwater fish do NOT drink. More about that at this link:

12.5. Fish Don’t Drink

It is easy to make medicated food. Heat 1/4 cup water (two ounces or 58 milliliters, not a lot) in the microwave. Then blend one 1/4 ounce of plain animal derived gelatin (Knox gelatin, one envelope) into the hot water with vigorous stirring. Take two tablespoons of dry commercial fish food (pellets or flake) and mix it with just a little of the hot water/ gelatin mixture. Add hot water/gelatin until you get a paste like consistency. If it gets too watery just add more food.

Then add just a “smidgen” (roughly 1/16 teaspoon, a 1% to 2% addition) of medication to the mud. If you are using more than one medication mix the medications together, then use just a “smidgen” of the mixture. If you are using a packet of medication, take just a “smidgen” of the packet contents. Mix and mash the whole mass thoroughly.  Spread it out into a pancake about 1/8th inch (3 mm) thick on a plastic film or a plate. Then put in the refrigerator. If you plan on keeping it for more than two weeks put it in a small plastic bag and freeze.

Treatment needs to be for at least two weeks. If it is one or two fish transfer the fish to a hospital aquarium and treat. If more than two fish have the problem one must treat the whole aquarium.

white spots on neons

Neon tetra disease with secondary bacterial infections (fin rot, saddleback probably caused by columnaris). Again note the “S” curve to the blue line, the extra white in the belly and the downward curve to the tail.

white spot on neon

Neon tetra disease with secondary bacterial infections (columnaris saddleback). Again note the “S” bend to the blue line and the extra white in the belly.


Note that there is a myth parroted over and over again that neons are dying because of inbreeding. You hear comments like: “Neon strains are so pathetically weak these days.” Mythbuster: neons are not weak because of breeding, they are just weak because they are a delicate blackwater species.

Neons have been bred in captivity but the captive gene pool that they originated from was very large. This makes it impossible for the fish to be inbred. There has also been no selection and breeding of brothers and sisters for many generations, which is the cause of inbreeding defects.

Short bodied fancy goldfish, balloon molly’s, electric blue Jack Dempsey’s and betta’s suffer from a host of problems due to inbreeding. This is not the problem with neons.

Another myth is that neons need acidic water. Or that they need water which is the same pH as the water they were born and raised in. Neons are bred in huge quantities in farms in Florida with hard water with a pH of 7.5 to 8.0 and do just fine. These fish will do well if put into an acidic environment even though raised in an alkaline environment. The breeders keep the water sparkling clear, very well aerated and bacteria free.

For an article about what is important and what is not important in water parameters click on this link:

4.1. Unimportant Water Parameters


Neon Tetra Disease in a Cardinal Tetra
Neon Tetra Disease and ich in a Cardinal Tetra

Cardinals are a fish obviously closely related to neons. But strangely enough, cardinals only rarely get neon tetra disease. But cardinals do occasionally get a very unusual disease. It is called dermocystidium. It causes large clear blisters to appear on the body of the fish. Some of the blisters appear to have white worms inside them.

Like neon tetra disease, this disease is incurable. Other tetras including neons can get this disease but the vast majority of the cases are cardinal tetras. Here is a fish with this rare disease:

dermocystidium on a cardinal fish
dermocystidium on a cardinal tetra
Website Articles on Neons

There are many websites out there telling everyone that neons are an easy to care for fish for beginners. Some of the articles are downright comical … or maddening .. or sad.

The website Aquarium Freaks has an article called “Neon Tetra care: the ultimate guide”. This article claims to be an expert on neons and says they are an easy fish for a beginner. The picture above the title is a cardinal tetra, not a neon. And the picture of a neon in the article is a neon with neon tetra disease. Here is the picture. Look at the head down swimming, the white area in the belly and along the spine, the slight downturn to the tail, and the crook in the blue line:

Fish with neon tetra disease
Fish with neon tetra disease


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