Columnaris

UrgentBacterial infection

Flavobacterium columnare

Also called: Cotton wool disease, Mouth fungus, Saddleback

What you see

Flat, pale patches, often with a yellowish or tan tinge, where the skin loses its shine — around the mouth, along the back (the classic saddle-shaped band at the dorsal fin), on fin edges and gills. A cottony growth can form around the mouth. It spreads fast — sometimes within a day or two.

How to tell it from its look-alikes

It differs from ich in shape: not discrete round grains but flat, spreading patches. From true fungus it differs three ways: the columnaris film lies flat rather than tufting upward, is often yellowish rather than grey-white, and spreads faster — visibly within a day or two, where fungus takes several days. Only a microscope separates these two for certain — so when in doubt, act as if it were columnaris, the faster and deadlier of the pair.

White spot (ich)

The separator is the shape of the spot: ich spots are discrete, round and raised, like grains. Velvet is far finer — not spots but a gold-coloured dust. Columnaris lesions are not spots at all but flat, yellowish, spreading patches. Male goldfish in breeding condition grow similar white tubercles on the gill plates — that is a sex trait, not a disease.

Fungus (saprolegnia)

True fungus almost always grows ON something — a wound, an injury, tissue that is already dead — and rises as a three-dimensional tuft. The columnaris film it is most confused with lies flat, is often yellowish, and spreads much faster. Only a microscope separates them for certain, so watch the speed: if the “fungus” spreads within a day, it is probably not fungus.

Fin rot

The three common causes leave different tracks. Tankmate nipping — clean, scissor-like notches with no eroding rim, and it continues only while the nipper is present (the planner warns which species nip fins). Bacterial rot — an uneven, crumbling edge with a rim, usually in poor water. And fins going together with slow wasting of the fish points to mycobacteriosis, which likes to impersonate fin rot. Which cause is yours is settled most reliably not by how the fins look but by a water test.

The dangerous mistake

Raising the temperature or treating it as a fungus. In warm water this bacterium turns far more aggressive — in experiments mortality climbs from near zero in cool water to total in warm. An antifungal does nothing against it: the familiar story is a few stable days of “treating fungus,” then sudden death.

Water first

In a young tank, most “disease” is not disease — it is ammonia or nitrite poisoning, and no medicine treats that. Before buying anything, test ammonia, nitrite and nitrate. If you keep the cycle log, the answer is already there.

New tank syndrome

What to do

Do not raise the temperature — if the tank runs warm, ease it down toward the low end your species still tolerate. Start an antibacterial ornamental-fish preparation immediately (sold in Lithuanian shops, e.g. sera Baktopur) and move the sick fish to quarantine if you can. If the gills are involved or it is moving through the group, you need a veterinarian — shelf products do not replace prescription medicine.

What to expect

The acute form kills within a day or two even under treatment. Caught early, while the lesion is small and local, it is curable — of all the diseases here, this is the one where acting the same day pays most.

This reference helps you recognise and tell conditions apart — it does not make the diagnosis. Only a microscope or a veterinarian confirms one. Use medicines exactly per the label, ideally in a quarantine tank, because several of them kill filter bacteria.