Flavobacterium columnare is present at low levels in many systems and becomes a disease when fish are stressed, crowded, injured or held in poor water. Its clinical importance comes from its speed and from a temperature relationship that runs opposite to a keeper's instincts: virulence increases with temperature, so warming a tank — the standard reflex for whitespot — actively worsens columnaris. Distinguishing the two is therefore not academic. The mouth presentation is widely called 'mouth fungus', which is wrong in a way that leads people to reach for the wrong product.
Short answer
Columnaris is a bacterial infection producing pale, greyish, flat lesions — classically a saddle-shaped band across the back, or a fraying, cotton-like growth around the mouth. It moves fast: in warm water it can kill within a day or two, and it moves faster the warmer the tank is, which reverses the usual advice about raising temperature.
Suggestive by eye, not conclusive
You can see it, but several conditions look like this. Work through the alternatives below before deciding what you are treating.
Water quality is the cause, the contributing factor or the deciding variable in the great majority of aquarium health problems. These steps come before any treatment decision, and for several conditions they are the entire treatment.
Every entry names the observation that separates it from this record. Work through these before deciding what you are treating — the most expensive mistake in fish health is confident treatment of the wrong thing.
How to tell it apart: Water-quality fin erosion progresses over weeks and improves on water changes alone. Columnaris progresses over hours to days and does not.
How to tell it apart: An injury is a single lesion in a location consistent with a specific cause — a heater, a sharp ornament, a jump — and it improves. Columnaris spreads and appears on other fish.
How to tell it apart: Water mould is a raised, soft, cotton-wool mass whose filaments visibly stream in the current. Columnaris lesions are flatter, greyer, more sharply bounded and often slightly sunken. This distinction matters because the treatments differ, and because the mouth presentation of columnaris is routinely mistaken for mould.
How to tell it apart: Velvet is a whole-body fine dusting with a gold tint. Columnaris is a localised lesion with defined margins.
Each entry states the mechanism, not just the association — an association without a mechanism is not something you can act on.
AquariumHQ names classes of response and never doses. Product concentrations differ between markets and formulations, several actives are temperature- and pH-dependent, and the invertebrate and plant tolerances that decide whether the rest of the tank survives are not printed on the label. Where a category puts a chemical in the water, what it can damage is stated with it.
Reducing temperature within the species' range slows the organism substantially. It is one of the few responses available quickly and without chemistry.
What it can damage: Cooling below a tropical species' range is itself a stressor and suppresses immune function. The target is the lower end of what the species tolerates, not cold water.
Correcting water quality and density removes the conditions the outbreak depends on. Without it, treatment addresses the symptom of a management problem.
Antibacterial treatment is frequently necessary given the speed of progression. Which agent is appropriate depends on the presentation and on what is lawfully available, and in many jurisdictions the effective options are prescription-only.
What it can damage: Antibacterial products suppress the nitrifying bacteria in the filter, and an ammonia spike during treatment is common and dangerous to fish already under stress. Several affect invertebrates. Test ammonia daily throughout.
Rapid progression and the prescription status of the effective agents in many countries make early veterinary contact worthwhile rather than a last resort.
AquariumHQ describes what is commonly reported and what husbandry correction is usually indicated. It does not diagnose and is not a substitute for an aquatic veterinarian. Many treatments are given to healthy fish suffering from a water-quality problem, which makes the outcome worse rather than better.
Published rather than hidden. A health page that lists no limitations has not had its limitations looked for.
Each claim is shown with what kind of support stands behind it and how strongly we hold it. Those are separate axes: a measurement held tentatively and an editorial position held firmly are different things.
Flavobacterium columnare, a filamentous gliding bacterium colonising skin, fins, mouth and gill tissue
Last verified 2026-08-11
Rate of progression and virulence increase with water temperature, so raising temperature — the standard response to whitespot — worsens this condition
This is the practical reason AquariumHQ treats the columnaris/whitespot distinction as one of the most consequential in freshwater fishkeeping.
Last verified 2026-08-11
The presentation commonly called 'mouth fungus' is a bacterial infection, not a fungal or oomycete one
Last verified 2026-08-11