Fungus and cotton-wool growth on a goldfish

A fuzzy white growth on a fish looks like the beginning of something. It is often the end of something — a wound, a scrape, a fin tear or a patch of dead tissue that has been sitting in water poor enough for moulds to take hold. That ordering matters, because it changes what you treat.

There is also a more urgent question hiding behind the first one, which is whether what you are looking at is fungus at all. The condition most often mistaken for it is bacterial, moves several times faster, and does not respond to anything sold for fungus.

First: is it fungus?

Three things are worth separating, and you can do it by looking.

True fungus — Saprolegnia and its relatives — is fuzzy and three-dimensional. It stands proud of the body in discrete tufts, like damp cotton wool, and it follows the outline of whatever damage is underneath it. It sits on a torn fin edge, a scrape, a missing scale, a rot lesion or an ulcer. It grows slowly, over days.

Columnaris is flat. It presents as a pale or greyish patch with a defined edge, often slightly raised at the margin, on skin that was not visibly damaged beforehand. Its favourite locations are the mouth — hence the old name, mouth fungus — the gills, and a saddle-shaped band across the back just in front of the dorsal fin. It spreads visibly from one day to the next, and an acute case can kill a fish within two or three days. On the lips it is one of several things the mouth problems page helps tell apart.

Breeding tubercles are neither. Small, hard, white pimples arranged evenly on a male’s gill covers and along the front edge of the pectoral fins, on a fish that is lively and probably chasing someone. They are firm, not fuzzy, and they are a seasonal feature rather than a problem — spawning covers the rest of that picture.

The practical test, in the absence of a microscope, is speed. Take a photograph today and compare it tomorrow in the same light. Fungus on a wound looks much the same. Columnaris does not.

Only a microscope settles it

Under a microscope the two are nothing alike: Saprolegnia is made of broad, branching filaments, and columnaris of masses of thin rods that gather in stacks. A vet can take a skin scrape and look, and that is the test that settles it. Lifting a fish out of the water to inspect the patch adds stress and tells you less than it seems to.

At home, judge by site and speed. Tufts standing on an obvious wound are fungus. A flat patch on unbroken skin, around the mouth, on the gills or as a saddle across the back, that is visibly larger from one day to the next is the one to treat as urgent.

Why it appears

Water moulds are permanent residents of every aquarium, present as spores in the water at all times, and they mostly settle on tissue that is already dead or damaged rather than attacking sound skin. A fish that is cold-stressed or run down can be colonised on live skin too, but that is the exception. That explains most of the pattern of the condition.

For fungus to appear, two things must usually both be true: there must be dead or badly damaged tissue for it to colonise, and conditions must favour the mould over the fish’s ability to heal. In practice that means the fish was injured — on decor, in a net, by a filter intake, during a spawning chase, by a parasite — and the water was poor enough that the wound did not close quickly.

This is why fungus is so strongly associated with neglected tanks, and why it is rare in well-maintained ones despite the spores being equally present in both.

What to do first

Test the water, then change a good proportion of it. If ammonia or nitrite reads above zero, or nitrate has climbed, the tank is both the reason the wound has not healed and the reason the mould established. Thirty to fifty percent, and at least half if ammonia reads 2 mg/L or more (unless the pH is below about 6, when smaller daily changes are safer), conditioned and temperature-matched, repeated daily while the readings are wrong. The testing guide explains what the numbers mean.

Hold the temperature steady. A fish healing tissue needs stability more than warmth, and a recent chill is a common precursor to the damage in the first place.

Find the injury. Look at the tank rather than only the fish: a sharp ornament, an unguarded filter intake, a rough net, a tankmate that has been chasing. If the cause is still there, the wound will keep reopening and the mould will keep returning no matter what you treat with.

Leave the growth alone. Picking or rubbing it off removes the mould and a layer of the fish with it, enlarging exactly the surface you are trying to close.

When to treat, and with what

Give clean water three or four days first, and judge by direction rather than presence: a shrinking patch on a fish that is eating and behaving normally needs nothing more. That is the majority of cases.

Where the growth is not shrinking, or where it is extensive, an antifungal treatment is reasonable — used properly. That means treating in a hospital tank rather than the display, removing activated carbon from the filter first or it will strip the medication, dosing against the tank’s real volume rather than the figure on its box, adding extra aeration because most treatments reduce dissolved oxygen, and finishing the course rather than stopping when the growth becomes less visible. Using medications safely sets all of that out step by step.

What not to do is work through a shelf. Each treatment stresses a fish that is already compromised, several damage the filter bacteria it depends on, and an antifungal used against columnaris does nothing while the real problem advances. The diseases reference sets out the distinction in more detail.

Fungus on the gills

The presentation nobody looks for, and the one that kills.

Moulds can colonise damaged gill tissue as readily as damaged skin, and the gills are both the most delicate tissue the fish has and completely hidden from view. A fish with gill involvement breathes fast, holds near the surface or the filter outflow, loses interest in food and becomes lethargic — and shows nothing at all on the outside.

That picture overlaps exactly with low oxygen, with gill flukes, with bacterial gill disease and with a nitrite problem — fast or laboured breathing is the page that sorts between them. The practical rule is the one from the gasping page: rapid breathing that does not improve when you add aeration, in water that tests clean, is a gill problem of some kind and needs identifying rather than guessing at. Gill conditions deteriorate faster than skin ones, and a visible external growth on a fish that is also breathing hard raises the priority considerably.

Why spring is the season for it

Pond keepers meet fungus far more often than indoor keepers, and often at the same time of year.

Over winter a pond fish runs cold. Its immune response is slowed to a fraction of its summer rate, it has not eaten properly for months, and its energy reserves are at their lowest. Meanwhile the water moulds are far less temperature-limited than the fish is. Any damage taken during winter — a scrape, a heron strike, a spawning injury from the previous autumn, frost damage to fins — sits in that mismatch for weeks.

The result is the classic spring fungus outbreak: fish emerging from winter with cotton-wool patches that were not there in November. The pond guide covers the seasonal routine; the relevant point here is that the fish most at risk is the one that went into winter in poor condition, which makes autumn feeding and autumn water quality a fungus prevention measure six months in advance.

Indoors, the equivalent is any tank that runs cold and unstable — a conservatory in February, an unheated room in a cold snap — where the same mismatch appears on a smaller scale.

Preventing it

Everything that prevents fungus is something you were doing anyway for other reasons.

Keep ammonia and nitrite at zero and nitrate down with regular water changes, so that a wound closes in days rather than sitting open. Remove anything sharp from the tank and guard the filter intake, so that fewer wounds happen at all — the decorations guide covers what is safe for long-finned fish. Keep the temperature steady. Avoid netting and handling except when necessary, because both strip the slime coat that is the fish’s first barrier. And put anything new through quarantine, since the parasites that break skin are a common first step on this road.

A goldfish in a well-run tank can lose a scale and never show a mark. The same injury in a neglected one is where a cotton-wool patch starts.