White patches on a fish: columnaris, true fungus and whitespot told apart

Quick answer

White or grey fuzz on a fish is not one disease. It is at least three, they progress at completely different speeds, and the popular first response to one of them accelerates another.

Columnaris is a bacterium, not a fungus. It moves fast, it moves faster in warm water, and it is the reason a fish that looked slightly off at breakfast can be dead by the following evening. True fungus is a water mould that only colonises tissue that was already damaged, and whitespot is a parasite with discrete, raised, salt grain spots.

• Grey or white film across the back in front of the dorsal fin, or around the mouth, with fins eroding from the edge inward: treat as columnaris and act today.
• Cotton wool tufts standing proud of a wound, an eye or a dead egg: true fungus, and the wound is the real problem.
• Discrete white dots like grains of salt scattered over the body and fins: whitespot, a parasite with a completely different treatment.
• Do not raise the temperature for a white patch you have not identified. Heat is the standard treatment for whitespot and it is close to the worst possible move for columnaris.
• Rapid breathing with pale, patchy gills and no external lesion at all can still be columnaris, in the form that kills fastest.

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Raising the temperature to treat a white patch can kill a tank in a day.

Flavobacterium columnare grows and spreads faster as water warms, and warm water simultaneously holds less oxygen at exactly the point where the gills are being destroyed. The whitespot protocol, raise the temperature and add salt, is standard advice in every fish forum and it is the wrong treatment here.

If you cannot tell columnaris from whitespot, do not use heat. Identify first.
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Decide in 60 seconds

Why the difference between bacterium and mould decides everything

Columnaris is caused by Flavobacterium columnare, a long, thin, gliding bacterium found in most freshwater environments. It is an opportunist. It lives in the biofilm of healthy tanks and only becomes a disease when the fish's defences drop.

The fish's first defence is the slime coat, a living antimicrobial layer. Anything that damages it opens the door: netting and shipping, a bite from a tankmate, ammonia or nitrite burn, a sudden temperature swing, crowding, high dissolved organic load, or low oxygen.

That is why columnaris so often appears within days of an event rather than out of nowhere. New fish, a house move, a filter failure, a heatwave, a spawning fight.

Because the bacterium is filamentous and gliding, the lesion it makes is flat. It looks like a film, a bloom, or a fading of colour, more like frost on a window than cotton wool. Under a microscope the organisms pile into the columns that gave the disease its name.

Saprolegnia and its relatives are not bacteria at all. They are water moulds, closer to algae than to true fungi, and they grow as branching filaments that stand up off the surface. Their crucial property is that they are saprophytes: they digest dead or badly damaged tissue.

That single fact changes the whole approach. Fungus on a fish is a sign that something already injured it. Killing the mould without finding the injury, or without fixing the water quality that let the injury go bad, treats the visible half of the problem.

Whitespot is different again. Ichthyophthirius multifiliis is a ciliated parasite that burrows into the skin and gill epithelium, and each visible dot is one parasite under the fish's own tissue. That is why the spots are raised, discrete and evenly sized, and why external medication only works during the free swimming stage of the life cycle, which is temperature dependent. Warming the water speeds that cycle up and shortens the window of vulnerability, which is exactly why heat is used.

Apply the whitespot logic to columnaris and you get the opposite result: you speed up the pathogen and slow down the fish.

What healthy skin, fins and mouth look like

What you are comparing against. Watch a fish through the glass at feeding time and learn its colour, its fin edges and how it holds itself, so a change registers on day one instead of day three.

Healthy skin has an even sheen and reflects light consistently along the flank. Colour is stable within the normal daily range for the species, and any breeding colour or stress striping comes and goes with context rather than persisting.

Fin edges are clean and translucent to the margin. The membrane between rays is intact, and the outer edge is a smooth line rather than a ragged or opaque one.

The mouth closes fully. The lips are the same colour as the surrounding skin, with no white rim, no grey film and no trailing tissue.

Breathing is even and symmetrical, both gill covers moving in the same rhythm. The rate itself varies enormously with species and temperature, so what matters is the change from that individual's own baseline.

The tell that separates early disease from normal variation is usually asymmetry or a hard edge. Natural colour changes are gradual and follow the fish's own markings. A lesion has a border.

Reading the lesion: the differential

Two failures show up repeatedly. The first is treating a flat grey lesion as fungus. The second is treating Epistylis as whitespot and concluding that the whitespot medication has failed.

Changes that mean act today

A lesion that has visibly changed size or shape since yesterday.

Speed is the most reliable field diagnostic available to an owner. Columnaris advances in hours in warm water. Fungus and lymphocystis do not.

Any involvement of the mouth.

Mouth lesions stop a fish eating, and they sit next to the gills. This is the presentation that most often ends with a dead fish two days after the owner decided to watch and wait.

Rapid or laboured breathing, especially with pale or blotchy gill tissue.

Gill columnaris destroys respiratory surface directly, and it can kill with almost nothing visible on the body. Increase aeration immediately, test ammonia and nitrite, and get advice the same day.

Two or more fish affected within a day of each other.

That is a system event. Something in the water changed, or something was introduced. Treating individuals will not fix it.

A recently added fish showing signs in its first fortnight.

Shipping stress and columnaris go together. This is the argument for a real quarantine period rather than a float-the-bag introduction.

What to do in the first hour, before any medication

The tools that actually matter: a soft net, a dosing syringe, and a clean container that will hold water taken from the tank. Nothing here involves a dry surface.

Increase aeration first. Every one of the likely diagnoses is made worse by low oxygen, and every treatment you might use lowers it further. An extra airstone costs nothing and buys time.

Stop feeding for the moment. Uneaten food raises the organic load, and dissolved organics are one of the conditions columnaris thrives in. A fish that is not eating is not going to starve in a day.

Test ammonia, nitrite, nitrate, pH and temperature, and write the numbers down before you change anything. If you treat first and test afterwards, you lose the information that would have told you what happened.

Check the temperature against the species range. If the reading sits at the top of the range and you suspect columnaris, bringing it down gradually towards the lower end slows the bacterium. Change it slowly, because a fast temperature drop is its own stressor.

Photograph the lesion in focus, from the side, in normal lighting, with something in frame for scale. Repeat the photograph twelve hours later. That pair of images tells a vet more than any description.

Only then decide about medication, and decide it with information about what is actually available where you live.

Treatment, and why the answer differs by country

Columnaris is a bacterial infection, so the definitive treatment is antibacterial. What that means in practice depends entirely on where you are.

In some countries, antibacterial fish treatments are sold over the counter in aquarium shops. In the United Kingdom, the European Union, Australia and many other places, antibiotics for fish are prescription-only and must come through a veterinarian. Products sold as fish treatments in those markets are usually antiseptics, dyes or antiparasitics rather than true antibiotics.

Several traditional aquarium chemicals are also restricted or withdrawn in some regions, and formulations change. Read the label of what you can actually buy rather than following a protocol written for another market.

This is why the practical route in most of the world runs through an aquatic veterinarian. Fish medicine is a genuine speciality, telemedicine consultations exist, and a vet can take a skin scrape and a gill biopsy and put them under a microscope. That distinguishes columnaris from Saprolegnia from Epistylis in minutes rather than by trial and error over days.

Where antibiotics are used, the choice ideally follows culture and sensitivity testing, because resistance in Flavobacterium is well recognised and a failed first course costs days the fish does not have.

Salt deserves a specific warning. It is the default aquarium remedy and it is not a reliable answer here. Many soft-water species, most catfish and loaches, and many plants tolerate it poorly, and it does not resolve an established bacterial infection.

Whatever you dose, remove activated carbon from the filter first, since carbon adsorbs medication, and calculate the dose from the real water volume rather than the tank's nominal size. Substrate, rock and equipment can displace a surprising fraction of a nano tank's stated volume.

What a vet will ask for, and what a diagnosis actually involves

A direct mouth examination is a clinician's procedure, done in seconds with wet hands. At home you check the same thing through the glass at feeding time: does the mouth close fully, and is there any grey film at the lips.

Expect to be asked for the tank volume, how long it has been running, the full stocking list, and the water test numbers rather than a description of them.

Expect questions about the last month: every fish, plant, snail, ornament or food added, every treatment used, any water change that went differently, any equipment that failed, and the temperature range across a day.

Expect to be asked how fast the lesion changed, which is why the twelve-hour photograph matters.

A vet may take a skin scrape and a wet mount from the edge of the lesion, and a small gill clip. Under the microscope, columnaris shows as slender rods forming haystack columns, Saprolegnia as broad branching filaments, Epistylis as stalked bell-shaped colonies, and whitespot as a large ciliate with a horseshoe nucleus. Each of those findings points to a different treatment.

If a fish has already died, a fresh body examined within hours is far more informative than a frozen one. Refrigerate, do not freeze, and take it in the same day if you can.

The routine that catches this early

What never to do

Never raise the temperature for an unidentified white patch. It is the correct move for whitespot and close to the worst move for columnaris, and the two are confused constantly.

Never treat a flat grey lesion with an antifungal and consider the matter handled. Antifungals have no effect on a bacterium, and the days spent waiting for them to work are the days that decide the outcome.

Never medicate the display tank without removing carbon and recalculating the real volume. Under-dosing selects for resistance and over-dosing crashes oxygen.

Never move a sick fish into a bare bucket of fresh tap water as a hospital tank. A hospital tank needs dechlorinated, temperature-matched, aerated water, ideally from the display tank, and it needs its own filtration or daily changes.

Never handle a fish on a dry surface. Dry contact strips the slime coat, which is the exact barrier this disease exploits.

Never assume a fish that has stopped showing lesions is clear. Survivors can carry the organism, and the tank they came from carries it too. What prevents the next outbreak is water quality, oxygen and reduced crowding rather than a completed course of anything.

When to get help

Contact an aquatic or exotics veterinarian the same day if any of these apply:

• A lesion involves the mouth or the gills
• Any fish is breathing hard, hanging at the surface, or holding a gill cover open
• A lesion has visibly grown within a day
• More than one fish is affected
• A fish has died and others in the same tank look off

Bring the water test numbers, the tank volume and age, the stocking list, everything added in the last month, the temperature range across the day, the twelve-hour photograph pair, and the label of anything you have already dosed.

If a fish has died, keep the body cold and take it with you. A fresh post mortem answers in an afternoon what watching the remaining fish will take a week to suggest.