Fish mouth rot or mouth fungus, sometimes known as cotton mouth or cotton wool disease, is one of the frequently seen fish diseases in home aquariums. It is so common that I ran into it during my first year of aquarium fish keeping.
Causes of Mouth Rot, Mouth Fungus on Fish
It is called mouth fungus by most people because sometimes it looks like mold or a cotton patch. It is, in fact, a bacterial infection caused by a type of bacteria called Flexibacter Columnaris or Flavobacterium Columnare.
There are bacteria everywhere; the aquarium water is no exception. In a well-maintained aquarium, these harmful bacteria are at very low concentration. They feed on organic waste such as fish poop and leftover fish food. Healthy fish have strong enough immune system to fight off the low concentration of harmful bacteria. However, if an aquarium is poorly maintained (e.g., too much fish poop and leftover fish food, or even a dead fish or two), there will be too many bacteria. And they can become opportunistic and expand their food source to the living tissues of the fish.
Since fish have no hands, they use their mouths to touch everything, to take food, and to fight (hopefully not). The most frequent injuries take place around their mouths. It is easier for the bacteria to get a foothold when there is an open wound. When it occurs, the fish’s mouth can at first look like having had some white lipsticks. When it gets worse, what looks like white or grayish mold or cotton will cover up fish lips. In more severe cases, you can see the fish lips are rotting away or falling apart.
Sometimes Columnaris can also infect the fish’s gills and fins. In those case, people call them gill rot and fin rot/fungus.
Fish mouth rot will not cure itself. If you do nothing, the fish will eventually die. The disease is especially common among the rainbow fish, as well as some catfish species. While it can be contagious, healthy fish in well-maintained aquariums might not be affected even if one or more infected fish are introduced to the same aquarium.
When an aquarium is in poor living conditions, (e.g., too many fish; poor water quality; low-quality fish food; lots of stress) it is very likely the disease can spread to the whole tank if it is not dealt with in time.
Treatment of Fish Mouth Rot (Fungus)
Since fish mouth rot is a bacterial infection, some general purpose anti-bacterial medications should work well treating the mild cases. In severe cases, an antibiotic should be used to control the infection.
Depending on the actual strain of the bacteria, some of them are resistant to one or more antibiotics. So far I have not heard any strain can resist all medications. Hopefully, it will never happen.
I bought Dwarf Neon Rainbow Fish with Mouth Fungus
When I ordered four Dwarf Neon Rainbowfish from a local pet store back in early 2011, they all came with mouth rot. I did not notice the mouth rot until I got home.
I have to note that I did not pick them out of the fish tank at the pet shop. It would be too careless of me if I did. The fish store did not carry them, so I pre-ordered them. I simply went to the store to pick them up once the shop staff called me. By the time I noticed the problem, I had already taken them home and set them free in my quarantine tank.
They all just looked like having white lips. I knew something was wrong just by the looks. After some research, I identified the problem as mouth rot. The good thing was that I had the quarantine tank. I had separated the four Dwarf Neon Rainbowfish from the rest of my fish in the beginning. It was a 10-gallon fish tank which had a cycled sponge filter and a heater.
Various online sources were quite confusing on the solution to cure fish mouth rot. Many different medications I have never heard of were recommended as cures. Some sources suggested dip, while some others went as far as suggesting injection or manually clean the fish’s lips with cotton balls soaked in meds.
After some debating, I decided I was not fit to be a surgeon. So I went for a “normal” medication treatment instead of the more troublesome methods.
The First Treatment: ParaGuard vs. Fish Mouth Rot
The first medication I tried was Paraguard from SeaChem. I had already had it at home. It said to be able to deal with various bacterial, fungal, and parasitic fish diseases. A general purpose medication for many fish problems. It is highly recommended by SeaChem to use on all new fish in a quarantine tank whether there is a problem or not. They said it is mild enough not to affect the fish and biological filtration. A great first try medication.
After more than a week of treatment with ParaGuard, the mouth rot had not gone away. A few of the Neon rainbowfish started to have their mouth rotting away. On the bright side, it also did not affect the biological filtration as I was testing the water frequently with API Freshwater Master Kit.
I went to SeaChem’s official forum and asked which product I should use to treat fish mouth rot. They told me usually Paraguard should fix the problem, but some strains of the bacteria can be resisting to some medications. I needed stronger medicines in this case. Then they went on and recommended KanaPlex.
The Second Treatment: KanaPlex vs. Fish Mouth Rot
I bought KanaPlex. It was a broad spectrum antibiotic. And it said to be able to treat various fish bacterial and fungal diseases. Both mouth rot and fin rot were under its treatable fish diseases in the product description. Furthermore, it claimed to be readily absorbed through both fish skin and gills. So ingestion is not required. However, staff from SeaChem recommended me to pre-soak fish food with KanaPlex for more effective treatment.
In addition to dose the meds in the water, I followed their instruction and pre-soaked all the dry food with it. I used high-quality New Life Spectrum pellets, pre-soaked with both VitaChem and Garlic Guard, on the top of KanaPlex of course.
It must be my bad luck. After more than six days of the recommended treatment period, the mouth rot did not go away. There was no sign of improvement at all.
On the other hand, the fish had become noticeably more healthy under my care. The rainbowfish all had dull colors when they first arrived. They were also shy and very inactive at first. They even spat out most of the food while ignored the rest for the first a few days. After just a few weeks at my home, they had become noticeably more colorful and active. They were actively chasing the food during every feed. The males even started to court the female all the time.
To my surprise, one day I started to notice some free swimming fry in the fish tank. They were so tiny, and they did not look like their parents at all. I guess I should have expected it since there were a female and three male, along with a big chunk of java moss in the quarantine tank as the perfect breeding bed. Oh yeah, the java moss was the key to breed Dwarf Neon Rainbowfish.
So I relocated the four adults Dwarf Neon Rainbowfish to a 5-gallon tank. To prevent the diseases passed to the fry. Nope, the adults did not eat the fry. They ignored them. On one occasion, I saw an adult rainbow mistakenly swallowed a fry. He immediately spat it back out. I guess they are smarter than average fish.
Back to the main topic.
I went to the SeaChem forum again. The staff was sorry to hear my results. KanaPlex should have worked as they said. They told me it could be the particular strain resist to KanaPlex. And they said Sulfathiazole should be the next one on the list to try.
The Third Treatment: Sulfathiazole vs. Fish Mouth Rot
Sulfathiazole is another antibiotic and anti-fungal medication. It claims to be highly effective treating bacterial infection. Fish mouth rot and fin rot were both on the list of treatable diseases in its description. So I ordered it online.
Since I have moved the four infected rainbowfish to the 5-gallon tank, I needed less dosage in the water to achieve the required concentration.
After just a few days of treatment, I could tell there was a noticeable improvement in the appearance of the fish lips. All visible symptoms had disappeared within one week. To make sure, I continued the treatment for two more weeks. It was probably unnecessary to keep the treatment for that long.
Updated on (12/9/2017): Now SeaChem sells a new product called SulfaPlex. The ingredient is 100% Sulfathiazole. They changed the name of their product. Sulfathiazole is the antibiotics, while SulfaPlex is the product name.
The aftermath of the Fish Mouth Rot Treatment:
Sulfathiazole had prevailed what others did not in my case. It did not mean other medications will be useless against all mouth rot cases. The particular strain of Columnaris on my fish had just happened to be resisting to ParaGuard and KanaPlex. It was still vulnerable to Sulfathiazole. It might just be the opposite outcome in different cases from other fish owners.
On the bright side, none of the fry (over 50 of them) was affected by either the disease or the medication. They all grew up nicely. I gave away 30+ small Neon Rainbowfish to friends when they reached the size of my thumbnail. The rest of them are living happily ever after in my community aquarium.
Prevention of Fish Mouth Rot:
Fish mouth rot/fungus is most likely to happen in a poorly maintained aquarium. I wonder what kind of miserable conditions my fish came from. The local pet store I bought them from was Pet Supplies Plus. They told me they got the fish from the same source where Walmart got their fish. No wonder! I need to remind myself never get fish from Walmart or any other shop who buy fish from the same source unless they improve.
Anyway, you can easily prevent it from happening by having a well-maintained aquarium. Mouth rot had never come back when the fish are under my care.
You can achieve this goal too by:
1. Having excellent water quality to keep the potentially harmful bacteria at the minimal level;
2. Feeding the fish with high-quality fish foods to enhance their immune system and overall health;
3. Keeping the fish stress free by not overstocking the aquarium; nor have some other stress factors such as incompatible fish species with aggression issues;
The Importance of a Quarantine Tank
I was able to prevent the spreading of the fish mouth rot from the start. Thanks to my quarantine tank. It also reduced the dosage of the medication since the quarantine tank is a lot smaller than the main tank.
If somehow you have a new fish or two with mouth fungus problem, you may just go for KanaPlex or SulfaPlex if you wish. Make sure to remove the carbon pad from your filter before the treatment. Activated carbon can absorb medications from the water to make the treatment ineffective.
Please note the Columnaris bacteria love warmer water especially within the 80~90F range. So keeping the water temperature at 70~75F during an outbreak can slow down the mouth rot considerably. Most tropical fish should be fine under this temperature, as long as you lower the water temperature slowly (no more than 3F per hour, the slower, the better). However, the too cold water temperature will lower the immune system of tropical fish. So you should avoid it too.
Please leave a comment if you have treated the fish mouth rot/fungus. I appreciate the feedback. 🙂 If you have a question, please make a thread on our forum. We can help you better in a forum thread than in the comment section.
I have a Neon Tetra with cotton mouth. I isolated it and have been treating it with Para Guard for ages and it is not getting better. If I put it back in the tank, will it infect the other fish?
Hi Paul,
Mouth rot has a high possibility to spread. Although, it depends on the immune system of individual fish and their overall health.
If ParaGuard is not working for you, you must switch to a different medication. Not all mouth rot is caused by the same strain of bacteria. You need one of the antibiotics mentioned in the article.
Good luck!
Hello; I have s red terror which I thought developed a fungal infection; white patchy fuzz over entire fish including mouth ; I treated with tetra fungus guard for 5 days … zero results . Then treated with API general cure for 4 days; zero results. Now using API Furan 2; results to be determined. If no luck, I will be starting API’s Sufa meds, per your success.
Thanks for your article … I hope it works!
Hi David,
A lot of the “fungal” cases are in fact a bacterial infection. Try antibiotics is the way to go. Sulfathiazole based medication cures both bacterial and fungal infection anyway. Good luck! Let me know if it worked. 🙂
Hello, I have a female Bettas who appears to have gotten cotton wool on her fin after getting an injury. She was immediately removed from the tank and put into quarantine (she’s in a 20gal sorority tank) with 1 tsp of aquarium salt per gallon. The mixture didn’t seem to do much after 2 days so we proceeded with a salt dip of 2.5 tsp of salt per gallon for 7.5 minutes. After she was placed into a tank with conditioned water, we noticed that the columnaris had started to pull up and the scales underneath were red and swollen. It’s been about 24 hours and now the scales look normal but the columnaris has not pulled up anymore nor has it reattached. Have you ever heard of this? Is there anything we can do to get it to pull up again without subjecting the fish to another salt dip? Would you recommend against any of the medications above because we have used salt?
She was in a 20 gal sorority, now she’s in a quarantine tank alone**
Salt might or might not slow down the infection, but it won’t cure it. You need one of the antibiotics if it is indeed the cotton wool disease. It is also not recommended to put a betta fish in a community tank, female or not.
One of my fish was infected by fungus… Others are also getting the same….after the treatment they are not recovering…..how to treat them… Which medicine should I use to treat them..
Hello Anushree,
Which medication did you use? How long have you been treating them?
It would be more helpful if you have provided some more details. Please come to the forum to make a thread with more details. We can help you better over there.
If you can not get treatment for the mouth fungus is there a natural/ treatment made from home that would work
Hello Kayla,
I am afraid not. Mouth rot/fungus is a bacterial infection and it does not just go away on its own. Antibiotics are needed to cure it.
What is the reason for not be able to get the medical treatment? Feel free to use our forum to start a thread if you have more questions.
I am dealing with the same issue now with tiger barbs.. from PET SUPPLIES PLUS! I will never buy fish from them again. I did introduce them to my main tank as well as ick.
Yeah, the source of fish is important. Sometimes it is worth to pay more to get better quality fish. Pet Supplies Plus gets their fish from the same source as Walmart, which is definitely cheap mass produced low-quality fish.
My gourami seems to have it inside her mouth as when breathing air from the surface (which she does a lot more frequently today than ever before) it pushes out the fish’s mouth (the cotton looking bacteria) then the fish seems to struggle and then sucks it back in. Should I try to remove the bit when the fish pushes it out as it seems to be blocking up the fish. Also has not been eating for a good 3-4 days I thought it was internal parasite but looks to be well hidden cotton mouth. Help!
Wow! Thank you do much! I have noticed one of my neon tetras has cotton mouth and I picked up ParaGard from my pet store. Then I noticed 2 more are just starting to get a very little spot on their lips so I have treated the tank but I have guppies in the tank and I dont see anyone of they with cotton moth. But I’m treating the 10 gal tank to be save. But do I need to take out my carbon filter with ParaGard? I am going to get the other Seachem product you mentioned. Just to be on the safe side. Sometimes it hard to find good info on-line. So I really appreciate your post.
Hi Brandy,
Thank you for reading.
Cottom mouth sometimes does not fully show until a few days after we got the fish. In my own case, they went from simply “white lips” to worse.
Paraguard is a general medication which might or might not be enough for the particular strain of the bacteria which is causing the mouth rot on your fish.
It is the best to remove the carbon pad in your filter during medication. Activated carbon is known to absorb meds in the water.
If you have further questions or would like to report your results with the medications, please feel free to use the forum. 🙂
Is it possible to do a trisulfa swab to treat the fungus rather than put the fish through being transferred to another tank?
I have one neon tetra that seemed to of developed it and one black phantom which may be developing it.
Hello. I have been searching everywhere to figure out how to treat my glofish tetra for a bump on its mouth. I do not think it is cotton mouth. It does not look a thing like cotton mouth examples in my opinion but I could be wrong. Regardless, I figured cotton mouth treatments would be similar. It is more of a very small flesh of the fish colored bump. Similar to a wary on a child’s finger if you have ever seen one. It is the color of my fish and very small. I did not even notice at first. I am on day 3 of using the Tetra Lifegaurd slow fizzing tabs. The fish in question looked very healthy and happy today. It was eating well, actively swimming and the growth did seem a tiny bit smaller. All of my 6 glofish tetras in a 10 gallon seemed much more active today. I have removed the carbon bag . I added API Stress Coat on day 1 and today which is day 3 of treatment. The last time my fish had ick which was 2 days after purchasing th the Tetra IckGaurd treatment knocked it right out. That is why I feel like this treatment is not working . Am I using the right product? Is the Stress Coat solution not needed? I’d like to add as little chemicals as possible so I will not add it again if it is indeed not really helpful right now
Thanks in advance!
I have tried every medication that you have listed in your article above to no avail. The sulfaplex has actually healed my goldfish a good percent, like maybe 75% of his mouth rot has been cured, but it just will not go away completely. I am now finishing up my second 10mg bottle of the medication (my goldfish is a 10 gallon quarantine tank). Is there any other medication that you know of that might work? I have spent so much money on this mouth rot and my poor fish has been suffering for about 7 months! PLEASE HELP!!!!
@ Danielle,
Sorry to hear that.
if you have tried every medication listed in the article and the mouth rot has not gone away, then there is a good chance that the particular strain of the bacteria has somehow developed resistance to them. Did you use the medication according to the instruction? It is quite important to follow the instruction and use the correct dosage. Please take this topic to the forum where it is easier for us to talk back and forth.
What is the best antibiotics for ornamental fishs
@ Sriram ram,
It depends on if your fish indeed have a bacterial infection. All those mouth rot, fin rot, etc. are caused by bacterial and thus antibiotics work.
You may try the medications I listed in the article. If it does not work, then try the next one.
I have 3 neon tetra that have this disease.
I separated the infected fish from other fish in the 55 gallon tank I have.
I put them in a sparse 20 gallon and tried the different reccomendations. The 3 tetra did not recover with medicine.
The medicine regime did bleach out a moss ball I had moved over in the hospital tank.
After eeaching a spending point on medicine that could have supplied a few school of tetra,
I decided that fish hospice was the best approach. I turned the hospital tank into a planted aquarium.
I assumed the 3 neons would pass away soon based on the warnings I read online and I would allow some time and water changes to pass before putting new fish in.
8 months have passed and those ugly little fish continue to live. They have to wait for food to start to fall to eat, but they hold on in their paradise tank.
Fortunately the vast number of plants help hide my ugly little monsters.
I don’t think that there is any estimate on how long the fish will hold on as they have well outlived dire mouth-rot predictions.
Two questions do spring up from this:
1. Does the moss ball have a chance at returning from its pale green color?
2. Considering how the fish are hanging on, should I start drinking the tank water in hopes of eternal life?
Hi. My java moss was not affected by the medication at all. That’s why my Neon rainbowfish even bred in the tank which the bottom was covered by moss. Different meds have different effects though.
Did you use all the recommendations? If not, which med did you use?
I still recommend fixing them up. It might be contiguous and spread to other fish tanks if you are not careful and using hands or tools for different tanks without sanitizing first.
I have 3 neon tetra that have this disease.
I separated the infected fish from other fish in the 55 gallon tank I have.
I put them in a sparse 20 gallon and tried the different reccomendations. The 3 tetra did not recover with medicine.
The medicine regime did bleach out a moss ball I had moved over in the hospital tank. (any chance for it to recover from the pale gree it changed to?)
After reaching a spending point on medicine that could have supplied a few school of tetra,
I decided that fish hospice was the best approach. I turned the hospital tank into a planted aquarium.
I assumed the 3 neons would pass away soon based on the warnings I read online and I would allow some time and water changes to pass before putting new fish in.
8 months have passed and those ugly little fish continue to live. They have to wait for food to start to fall to eat, but they hold on in their paradise tank.
Fortunately the vast number of plants help hide my ugly little monsters.
I don’t think that there is any estimate on how long the fish will hold on as they have well outlived dire mouth-rot predictions.
I want to move my gourami over to that tank to end an ongoing feud with an anglefish, don’t have the heart to euthanize the tetra.
I wouldn’t euthanize the tetra either. I asked in the other reply which med did you use? It took me a few tries to fix my fish.
By the way, if you make threads on the forum, it would be easier for us to go back and forth discussions.