Successful Treatment of Fish Mouth Rot (Mouth Fungus; Cotton Mouth; Columnaris) on My Dwarf Neon Rainbow Fish

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 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.  

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.

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. Basically, it was 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. 

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.  

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 week 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.  

Note 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 could not in my case. It did not mean other medications will be useless against all mouth rot cases. The particular strain of Columnaris, 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. 

Anyway, you can easily prevent it from happening by having a well-maintained aquarium. In fact, 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 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 cut down 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 want to try one of the medications I have used earlier. 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).

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2 thoughts on “Successful Treatment of Fish Mouth Rot (Mouth Fungus; Cotton Mouth; Columnaris) on My Dwarf Neon Rainbow Fish

  1. 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!

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