Columnaris: the cotton mouth that is not a fungus, and moves fast
Grey-white patches on the mouth or a saddle across the back: columnaris is a fast bacterial infection, not fungus. How to tell them apart, and how to treat it.
By Xavier Jodoin
In short
Columnaris is a bacterial infection, not a fungus, and it can kill in two to three days in warm water. It shows as grey-white, flat patches on the mouth, gills or back, a pale saddle across the body, and fins fraying fast. Move the sick fish to a hospital tank, lower the temperature to 24–25 °C, add aquarium salt at one tablespoon per 20 litres, and treat with an antibacterial — a kanamycin-and-nitrofurazone combination is the standard — for the full course. Fix the water in the main tank, because that is where it took hold.
What you are looking at
A pale, greyish-white patch — on the lips and around the mouth most often, which is where the name “cotton mouth” comes from, or on the gills, or as a band across the back behind the dorsal fin, the “saddleback”. The edges of the patch may be red. Fins go ragged and shrink within days rather than weeks, the fish breathes fast if the gills are involved, and it stops eating. Livebearers, tetras, rainbowfish and any fish fresh from a shop or a shipping bag are the usual victims, and it often follows a temperature spike or a week of poor water.
Columnaris is a bacterium, Flavobacterium columnare, and the point of this guide is that it is not a fungus, although it is sold as one and treated as one by almost every beginner, because the patches look cottony. The difference is the texture, and it is worth ten seconds with a torch. A true fungus — Saprolegnia, which grows on wounds and dead tissue — is fluffy, raised, three-dimensional, like cotton wool stuck to the fish. Columnaris is flat, or nearly: a dull, slightly fuzzy film that looks more like a scuff than a growth, and it sits on the mouth, gills and back, where a true fungus rarely starts. A fungus treatment does nothing to columnaris, and columnaris does not wait.
The other tell is speed. In warm water — 28 °C and above — columnaris can take a fish from a pale lip to dead in two or three days, and the rest of the tank a few days after. It thrives in warmth, in high organic waste, in low oxygen and in crowded tanks, and it spreads through the water and by contact, so this is one of the few diseases in these guides that is genuinely contagious and genuinely urgent.
The treatment
1. Move the sick fish, today. A bare 10–20 litre hospital tank with a heater, an air stone and a sponge filter. Not for the fish’s comfort, though that too — for the rest of the tank, which is not yet sick. Watch the others closely for a week; any that show a patch join it.
2. Cool the water. This is the step that is backwards from every other disease guide on this site. Columnaris multiplies fastest in warmth, so bring the hospital tank — and the main tank, if the fish in it tolerate it — down to 24–25 °C, a degree every few hours. Do not heat a columnaris tank to “speed the treatment”; that is the ich playbook and it makes this one worse.
3. Oxygen. An air stone running hard, or the filter outlet churning the surface. The bacteria attack the gills and dislike well-oxygenated water; the fish needs every bit of it.
4. Aquarium salt. One tablespoon per 20 litres (5 gallons), dissolved first, in the hospital tank. It slows the bacteria and supports the slime coat, and it is safe for the fish that get columnaris most. Skip it for scaleless species.
5. An antibacterial, the right one, the full course. Columnaris is a gram-negative bacterium, and the treatment that works is a combination of kanamycin and nitrofurazone (sold as separate products, and safe to use together) dosed for the hospital tank’s actual volume, with the activated carbon out of the filter. Where those are not sold, a nitrofurazone or oxytetracycline product on its own is the next best. Erythromycin, the common “fin rot” antibiotic, does not touch it. Complete the course on the label, usually five to seven days, with a 25–50 % water change before each re-dose. A fish that is still eating can also take the medication in food, which reaches an internal infection better than the water does.
6. Dips for a fast case. A fish going downhill by the hour can have a salt dip — one tablespoon of aquarium salt per litre, for five to ten minutes, taken out the moment it rolls or gasps — or a potassium permanganate dip by the label. Dips knock the bacteria back on the surface while the antibiotic in the tank does the slow work.
In the app — a checkup with a photo of the patch and your readings tells you whether this is columnaris or a true fungus, which changes the treatment completely, and gives a plan sized to your hospital tank’s volume; Add plan to my schedule books the daily water changes and the medication days, and the follow-up asks whether the patch is shrinking.
The main tank
Columnaris is in every aquarium at a low level, and it took hold in yours because something let it. Test ammonia, nitrite, nitrate, pH and temperature in the main tank now. A 30–50 % water change if anything reads wrong, and a 25 % change every two or three days for a fortnight in any case, with the gravel vacuumed — the bacteria live in organic waste. Turn the heater down to 25 °C if the stock tolerates it, feed lightly, and check the tank is not overstocked: crowding is the condition columnaris likes best. If more than one fish is showing patches, treat the whole tank with the same antibacterial rather than moving fish one by one.
In the app — log the test and the verdict says which readings let this happen; the schedule advisor suggests testing every two days until they clear, and the water-change calculator sizes each change to your tank.
Afterwards
A fish that is eating again with the patch shrinking and the fins no longer receding has turned the corner; the fins regrow over weeks, as after fin rot. Keep it in the hospital tank until the patch is gone and a week beyond. Every fish that arrives from now on spends four weeks in quarantine — columnaris travels in the shop’s water and on stressed new fish more than any other way, and a fortnight of watching a new fish’s lips is how you never see it in the display tank again.
Common questions
Is columnaris a fungus?
No. It is a bacterium, Flavobacterium columnare, that happens to look cottony. A true fungus is fluffy and raised, like cotton wool on a wound; columnaris is a flat, dull, greyish film on the mouth, gills or back. Fungus treatments do nothing to it, and it moves far faster than a fungus, so getting this right on day one matters.
How fast does columnaris kill?
In warm water, 28 °C and above, a virulent strain can kill a fish in two or three days and spread through a tank within a week. In cooler water it is slower, which is why lowering the temperature to 24–25 °C is part of the treatment. Treat it the day you see it, not after a weekend of watching.
Is columnaris contagious?
Yes, unusually so for a fish disease. It spreads through the water and by contact, and it thrives in crowded, warm, waste-rich tanks. Move the sick fish to a hospital tank the same day, watch the rest closely, and treat the whole tank if more than one fish shows a patch.
What is the best treatment for columnaris?
A combination of kanamycin and nitrofurazone, dosed for the tank’s actual volume with the carbon removed, for the full course on the label, in water cooled to 24–25 °C with an air stone running and aquarium salt at one tablespoon per 20 litres. Erythromycin and fungus treatments do not work on it. A salt dip helps a fish that is going downhill by the hour while the antibiotic does the slower work.