Fish mouth rot or mouth fungus, also known as cotton mouth or cotton wool disease, is a common fish disease in home aquariums. It’s so common that I encountered it during my first year of keeping aquarium fish.
- Causes of Mouth Rot, Mouth Fungus on Fish
- Fish Mouth Rot Treatment (Fungus)
- The Aftereffects of Fish Mouth Rot Treatment:
- Fish Mouth Rot Prevention:
- The Importance of a Quarantine Tank
Causes of Mouth Rot, Mouth Fungus on Fish
Most people refer to it as mouth fungus because it can resemble mold or a cotton patch. It is, in fact, a bacterial infection caused by Flexibacter Columnaris or Flavobacterium Columnare bacteria.
Bacteria can be found in almost any environment, including aquarium water. These harmful bacteria are present in very low concentrations in a well-maintained aquarium. They eat organic waste like fish poop and leftover fish food. A healthy fish’s immune system is strong enough to fight off the low concentration of harmful bacteria. However, if an aquarium is not properly maintained (e.g., too much fish poop and leftover fish food, or even a dead fish or two), there will be an overabundance of bacteria. They can also become opportunistic and expand their food source to the fish’s living tissues.
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Because fish lack hands, they use their mouths to touch everything, eat, and fight (hopefully not). The most common injuries occur around their mouths. When there is an open wound, it is easier for bacteria to get a foothold. When this happens, the fish’s mouth may appear to be covered in white lipstick at first. When the situation worsens, what appears to be white or grayish mold or cotton will cover up the fish lips. In more severe cases, the fish lips can be seen rotting or falling apart.
Columnaris can sometimes infect the fish’s gills and fins. In those cases, they are referred to as gill rot and fin rot/fungus.
Fish mouth rot will not go away on its own. If nothing is done, the fish will perish. Rainbow fish, as well as some catfish species, are particularly susceptible to the disease. While it is contagious, healthy fish in well-maintained aquariums may not be affected even if infected fish are introduced to the same aquarium.
When an aquarium has poor living conditions (e.g., too many fish; poor water quality; low-quality fish food; a lot of stress), it is very likely that the disease will spread to the entire tank if not treated promptly.
Fish Mouth Rot Treatment (Fungus)
Because fish mouth rot is a bacterial infection, some over-the-counter anti-bacterial medications should be effective in treating mild cases. An antibiotic should be used to control the infection in severe cases.
Some bacteria, depending on their strain, are resistant to one or more antibiotics. So far, I haven’t heard of any strain that can withstand all medications. Hopefully, it won’t happen.
Dwarf Neon Rainbow Fish with Mouth Fungus was purchased by me.
Back in early 2011, I purchased four Dwarf Neon Rainbowfish from a local pet store, and all of them had mouth rot. I didn’t notice the rot in my mouth until I got home.
I should point out that I did not choose them from the pet store’s fish tank. It would be irresponsible of me to do so. I pre-ordered them because the fish store didn’t have them. When the shop staff called me, I simply went to the store to pick them up. I had already taken them home and set them free in my quarantine tank by the time I realized there was a problem.
They all appeared to have white lips. I could tell something was wrong just by looking at her. After some investigation, I determined that the issue was mouth rot. The quarantine tank was a blessing in disguise. In the beginning, I separated the four Dwarf Neon Rainbowfish from the rest of my fish. It was a 10-gallon fish tank with a sponge filter that cycled and a heater.
Various online sources provided conflicting information about how to treat fish mouth rot. Many medications that I had never heard of were recommended as cures. Some sources suggested dipping, while others suggested injection or manually cleaning the fish’s lips with med-soaked cotton balls.
After some deliberation, I concluded that I was not qualified to be a surgeon. So, instead of the more difficult methods, I chose a “normal” medication treatment.
The First Treatment: ParaGuard vs. Fish Mouth Rot
The first medication I tried was SeaChem’s Paraguard. I had it at home already. It is said to be effective against a variety of bacterial, fungal, and parasitic fish diseases. A multi-purpose medication that can be used to treat a wide range of fish problems. SeaChem recommends using it on all new fish in a quarantine tank, whether there is a problem or not. They claim it is gentle enough not to harm the fish or biological filtration. A fantastic first-time medication.
The mouth rot remained after more than a week of treatment with ParaGuard. The mouths of a few of the Neon rainbowfish began to rot. On the plus side, because I was testing the water frequently with the API Freshwater Master Kit, it had no effect on the biological filtration.
I went to the official SeaChem forum and asked which product I should use to treat fish mouth rot. They told me that normally, using Paraguard should solve the problem, but that some strains of the bacteria may be resistant to certain medications. In this case, I needed stronger medications. They went on to recommend KanaPlex.
The Second Treatment: KanaPlex vs. Fish Mouth Rot
KanaPlex was purchased by me. It was an antibiotic with a broad spectrum of action. It is also said to be capable of treating a variety of fish bacterial and fungal diseases. In the product description, both mouth rot and fin rot were listed as treatable fish diseases. It was also claimed to be easily absorbed through fish skin and gills. As a result, ingestion is not required. However, SeaChem staff advised me to pre-soak fish food in KanaPlex for more effective treatment.
In addition to dosing the medications in water, I followed their advice and pre-soaked all of the dry food with it. On top of KanaPlex, I used high-quality New Life Spectrum pellets that had been pre-soaked with VitaChem and Garlic Guard.
It’s got to be my bad luck. The mouth rot persisted after more than six days of the recommended treatment period. There was no indication of progress.
The fish, on the other hand, had become noticeably healthier under my care. When the rainbowfish first arrived, they were all dull in color. They were also initially shy and inactive. For the first few days, they even spat out most of the food while ignoring the rest. They had become noticeably more colorful and active after only a few weeks at my house. During each feeding, they were actively chasing the food. The males even began to court the females on a regular basis.
Surprisingly, I began to notice some free swimming fry in the fish tank one day. They were so small, and they didn’t resemble their parents at all. I suppose I should have expected it given that there was a female and three males in the quarantine tank, as well as a large chunk of java moss as a perfect breeding bed. Oh, and the java moss was essential for breeding Dwarf Neon Rainbowfish.
As a result, I moved the four adult Dwarf Neon Rainbowfish to a 5-gallon tank. To prevent diseases from being passed on to the fry. No, the adults did not consume the fries. They were completely ignored. On one occasion, I observed an adult rainbow swallowing a fry by mistake. He spit it out right away. They must be smarter than the average fish.
Let’s get back to the main point.
I returned to the SeaChem forum. The staff was disappointed with my results. KanaPlex should have performed as advertised. They suggested that it could be the strain’s resistance to KanaPlex. Sulfathiazole, they said, should be the next one on the list to try.
The Third Treatment: Sulfathiazole vs. Fish Mouth Rot
Another antibiotic and antifungal medication is sulfathiazole. It claims to be extremely effective in the treatment of bacterial infections. Both fish mouth rot and fin rot were listed as treatable diseases in the description. As a result, I ordered it online.
I needed less dosage in the water to achieve the required concentration since I moved the four infected rainbowfish to the 5-gallon tank.
I could tell there was a noticeable improvement in the appearance of the fish lips after only a few days of treatment. Within one week, all visible symptoms had vanished. I continued the treatment for two more weeks just to be sure. Keeping the treatment for that long was probably unnecessary.
Updated on December 9, 2017: SulfaPlex, a new product from SeaChem, is now available. Sulfathiazole is the sole ingredient. Their product’s name was changed. The antibiotic is sulfathiazole, and the product is called SulfaPlex.
The Aftereffects of Fish Mouth Rot Treatment:
Sulfathiazole had won where others had failed in my case. It does not imply that other medications will be ineffective against all cases of mouth rot. The strain of Columnaris on my fish happened to be resistant to ParaGuard and KanaPlex. It remained susceptible to Sulfathiazole. It is possible that the opposite outcome will occur in different cases from other fish owners.
On the plus side, neither the disease nor the medication had any effect on the fry (there were over 50 of them). They all matured well. When they reached the size of my thumbnail, I gave away 30+ small Neon Rainbowfish to friends. The rest of them are happily ever after in my neighborhood aquarium.
Fish Mouth Rot Prevention:
Fish mouth rot/fungus is most likely to occur in an unclean aquarium. I’m curious as to what kind of deplorable conditions my fish came from. Pet Supplies Plus was the local pet store where I purchased them. They said they got the fish from the same place Walmart got theirs. It’s no surprise! I need to remind myself that I should never buy fish from Walmart or any other store that buys fish from the same source unless they improve.
In any case, having a well-maintained aquarium can easily prevent this from happening. Mouth rot had never returned when the fish were in my care.
You, too, can achieve this goal by:
- Having high-quality water to keep potentially harmful bacteria to a minimum;
- Feeding high-quality fish foods to the fish in order to boost their immune systems and overall health.
- Keeping the fish stress-free by not overstocking the aquarium, as well as avoiding other stressors such as incompatible fish species with aggressive tendencies.
The Importance of a Quarantine Tank
I was able to stop the spread of fish mouth rot from the beginning. Because of my quarantine tank. Because the quarantine tank is much smaller than the main tank, the medication dosage was also reduced.
If you have a new fish or two with a mouth fungus problem, you can simply use KanaPlex or SulfaPlex. Before beginning the treatment, remove the carbon pad from your filter. Activated carbon has the ability to absorb medications from water, rendering the treatment ineffective.
Please keep in mind that the Columnaris bacteria prefer warm water, preferably between 80 and 90 degrees Fahrenheit. Keeping the water temperature at 7075F during an outbreak can significantly slow down the mouth rot. Most tropical fish should be fine at this temperature if the water temperature is gradually reduced (no more than 3F per hour, the slower, the better). Tropical fish’s immune systems will be compromised if the water temperature is too cold. As a result, you should avoid it as well.
If you have treated the fish mouth rot/fungus, please leave a comment. I appreciate your feedback. If you have a question, please post it in our forum. We can assist you more effectively in a forum thread than in the comment section. Fish mouth rot or mouth fungus, also known as cotton mouth or cotton wool disease, is a common fish disease in home aquariums. It’s so common that I encountered it during my first year of keeping aquarium fish.