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 personally ran into it during my first year of aquarium fish keeping.
Causes for 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 in very low concentration. They feed on organic waste such as fish poop and leftover fish food. Healthy fish also have strong enough immune system to make low concentration of these bacteria harmless. 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 on the fish.
Since fish have no hands, they use their mouths to touch everything, to take food, and to fight (hopefully not). The most frequently injuries take place on their mouths. It is easier for the bacteria to get a foothold there when there are open wounds. When this occurs, the fish’s mouth can at first look like having had some white lipsticks. When it gets worse, fish lips can be covered by what looks like white or grayish mold or cotton. In more severe cases, you can literally see the fish lips are rotting away or falling apart. Sometimes columnaris can also infect the fish’s gills and fins. In those case, they are called gill rot and fin rot/fungus.
Fish mouth rot will not cure itself. If nothing is done, 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. However, 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 caused by bacteria, some general purpose anti-bacterial medications should work well treating 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, but so far I have not heard any strain can resist all medications.
When I ordered four Dwarf Neon Rainbow fish back in 2010, they all came with mouth rot. I did not notice it until I got home. I have to note that I did not pick them out of the fish tank. It would be too careless of me if I did. The fish store did not carry them, so I pre-ordered them and went to the store to pick them up. At the time I noticed the problem, 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 a quarantine tank. The four Dwarf Neon Rainbow fish were separated from the rest of my fish from the beginning. It is a 5-gallon fish tank which had a cycled sponge filter and a heater.
Various online sources are quite confusing about 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 to go for a “normal” medication instead of the more troublesome methods.
First Treatment: ParaGuard vs. Fish Mouth Rot
The first medication I tried was Paraguard from SeaChem. It is 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. I picked it first because it is said to be mild enough to not affect the fish and biological filtration. In fact, it is highly recommended by SeaChem to use on all fish in a quarantine tank whether there is a problem or not. After more than two weeks of treatment with ParaGuard, the mouth rot had not gone away. A few of the Neon rainbow fish started to have their mouth rotting away. On the bright side, it also had no effect on the biological filtration as I was testing the water frequently with API Freshwater Master Kit.
To my surprise, I started to notice some free swimming fry in the fish tank. 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. So I removed all the fry and java moss to a separate tank with no other fish.
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 certain medications. Stronger medicines were needed in this case. Then they went on and recommended both SulfaPlex and KanaPlex.
Second Treatment: SulfaPlex vs. Fish Mouth Rot
I picked SulfaPlex first, another antibiotic and anti-fungal medication. It contains 67% sulfathiazole which was said to be highly effective treating bacterial infection. Fish mouth rot and fin rot were both listed in its description as treatable diseases. Due to bad luck, it did not work for me. Just like ParaGuard, it did not affect the biological filtration.
Third Treatment: KanaPlex vs. Fish Mouth Rot
I finally ordered KanaPlex. It was a broad spectrum antibiotic, and it said to be able to treat various bacterial and fungal fish diseases. Once again, both mouth rot and fin rot were listed under its treatable fish diseases in the product description. Furthermore, it was said to be easily absorbed through both fish skin and gills. So ingestion is not required. After just a week of treatment, I could tell there was noticeable improvement by the appearance of the fish lips. All visible symptoms were gone within two weeks. To be on the safe side, I continued the treatment for another two weeks although it was probably unnecessary to do that long.
Aftermath of the Fish Mouth Rot Treatment:
KanaPlex had prevailed what others could not in my case. It did not mean other medications will be absolutely useless against all mouth rot cases. The particular strain of Columnaris just happened to be resisting to ParaGuard and SulfaPlex, while still vulnerable to KanaPlex. It might just be the opposite outcome for someone else.
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 Rainbow fish to friends, and kept the rest. Now they are all living happily after in my community aquarium.
Prevention of Fish Mouth Rot:
As was mentioned, fish mouth rot / fungus is most likely to happen in a poorly maintained aquarium. The opposite can be true that you can easily prevent it from happening by having a well maintained aquarium. As a matter of 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 potential 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 stressful factors such as incompatible fish species with aggression issues.
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 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 70~80F 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).