Columnaris in fish: cotton wool mouth and saddleback, and how to treat it

Columnaris is a fast bacterial disease that causes white fuzz on the mouth, pale patches, or a pale "saddle" across the back, and it can kill fish within hours to a day or two. The most important thing to do is remove dead fish, clean up the water, and contact a vet or experienced fish store today, because it often needs an antibiotic.

It's caused by Flavobacterium columnare. In the aquarium trade it's often called "cotton wool disease" or "mouth fungus", even though it isn't a fungus at all (Declercq and others, 2013). It affects many freshwater fish, including livebearers, goldfish, catfish and cichlids. The bacteria attack the skin, fins, mouth and gills, and gill damage often kills fish before you see much on the outside. Columnaris loves warm water and stress: the same review found warmer water helped the bacteria stick to gills and spread, and outbreaks often follow shipping, crowding, poor water or handling.

How to identify it

Pale, greyish or whitish patches on the skin, often on the back near the dorsal fin, as if the colour has faded.
A white or yellowish rim or fuzz on the lips.
Frayed fins with a whitish edge.
The classic "saddleback": a pale band across the back, like a saddle.

Easily confused with:

True fungus (water mould): fluffy, long white strands, usually growing on an existing wound or dead tissue. Columnaris lesions are flatter, more matted, and spread fast on fish that weren't injured.
Fin rot: slower, mostly on the fin edges. Columnaris can also cause fin rot, so they overlap.
Ich (distinct white dots) and mouth injuries from fighting or jumping.

A vet can confirm columnaris quickly by looking at a skin or gill scrape under a microscope: the bacteria form typical "columns" or haystacks (Merck Veterinary Manual).

When it's a problem

Early: pale patches, a white rim on the lips, frayed fins, faster breathing. Act today; bath treatments work best at this stage.
Serious: a clear saddleback; the mouth eroded or covered in cottony, yellow-white growth (these fish often stop eating and starve); ulcers that may reach the muscle, often covered in yellowish mucus.
Emergency: gasping, then deaths, sometimes many in a day or two, or lesions spreading overnight. High-virulence strains kill within hours to a day or two.
Normal, don't worry: there's no harmless form, but milder strains do cause slower, creeping infections that give you more time.

What to do right now

Remove any dead fish immediately. The review found that dead fish spread columnaris faster than living ones.
Test the water: ammonia, nitrite, pH, temperature. Fix anything that's off.
Do a 30–50% water change with dechlorinated, temperature-matched water, and vacuum the gravel.
Add aeration. Columnaris attacks gills.
Don't raise the temperature. This is the opposite of ich. Heat helps columnaris. If your tank is at the warm end of your fishes' range, some keepers lower it by 1–2°C (2–4°F), slowly, staying inside what your species need.
Isolate badly affected fish in a hospital tank if you have one, and don't share nets.
Call a vet or experienced fish store today. Columnaris often needs an antibiotic, and in many countries that means a prescription. Starting the process early matters.

What to do next

Water quality and stress first

Columnaris thrives on organic waste. The review notes that the bacteria grow on leftover food and survive longer in water rich in organic matter. Frequent water changes, gravel vacuuming, light feeding, and fewer fish per tank all help. This alone won't cure an aggressive outbreak, but no treatment works well without it.

Salt

Salt has some evidence behind it. Studies summarised in the review found lower death rates as salinity rose to a few grams per litre. Many tropical fish tolerate 1–3 g per litre for a treatment period. But plecos, other catfish, loaches, and soft-water fish may not, and experts disagree on how safe salt is for them. Many plants and snails don't tolerate it either. Salt is a support, not a cure.

Antibiotics

Moderate and severe columnaris usually needs an antibiotic. Medicated food reaches the bacteria inside the fish if the fish is still eating; bath treatments work best early, before the infection goes deep (Declercq and others).

Many countries require a vet prescription for fish antibiotics, and the rules have been tightening. Check what applies where you live.
Columnaris strains from ornamental fish have shown resistance to several common antibiotics. A vet can culture the bacteria and pick a drug that works. That's better than trying three shop products in a row.
Some antibiotics in the water harm the filter bacteria. Test ammonia and nitrite every day or two.
Remove activated carbon during treatment, follow the label or your vet's directions, and finish the full course even if the fish look better.

Other treatments

Fish farms use potassium permanganate baths, and some aquarium products use antiseptics such as acriflavine. These can help in early cases but are harsh on gills, plants and invertebrates, and dosing errors are easy. Use them only with clear instructions.

Cautions

Shrimp and snails: many antibacterials and permanganate harm them. Remove them first if you can.
Catfish, plecos, loaches and scaleless fish: often more sensitive to medicines and salt.
Live plants: permanganate and salt can damage them.

How long it lasts

With effective treatment, you should see no new lesions within a few days and slow healing over 1–3 weeks. Keep water changes going for at least two weeks after the last sign. Fish that survive can carry the bacteria and shed it later (the review calls them carriers), so watch the tank closely next time something stresses it.

Products that help

Liquid test kit for ammonia, nitrite and pH: to find and fix water problems now, and to watch the filter during antibiotic treatment.
Water conditioner that removes chlorine: for the frequent water changes columnaris treatment needs.
Gravel vacuum: removes the organic waste the bacteria feed on.
Air pump and air stone: extra oxygen for damaged gills.
Aquarium salt (or non-iodized salt with no additives): a support for salt-tolerant fish, not a cure; skip it for sensitive species and planted tanks.
Antiseptic medication containing acriflavine: can help early cases, if you follow the instructions closely.
Medicated (antibiotic) fish food or antibiotic bath: for moderate and severe cases, usually through a vet.
Hospital tank: a separate tank for isolating badly affected fish.

Keep on hand before trouble starts: a water test kit, water conditioner, an air stone, and the phone number of a vet who sees fish.

How to prevent it

Quarantine new fish for at least 4 weeks.
Keep stocking moderate and water clean: weekly tests and water changes.
Avoid running tropical tanks hotter than your fish need.
Remove dead fish at once. Disinfect nets between tanks.

Why it happened

Stress: shipping, crowding, rough handling, fighting.
Warm water at the top of the fish's range.
Poor water: ammonia, nitrite, low oxygen, lots of waste. See overfeeding.
A carrier fish added without quarantine. Livebearers from mass farms are a common source.

When to get help

Columnaris is one of the conditions where a vet visit is most worth it, because the right antibiotic matters and resistance is common. Get help the same day if fish are dying, if the mouth is affected, or if lesions spread overnight. A good local fish store can tell you whether a fish vet works nearby. Honest limit: in fast outbreaks, you may lose fish no matter what you do. Focus on saving the rest.

In the game

In Multiple Tank Syndrome, the Columnaris card ("cotton-wool mouth") kills half the fish in the hit tank at random, or every fish if the tank is 28°C or warmer, because real columnaris is worse in warm water. Antibiotic food or the Medicine basket saves the tank.