Fish fungus and white fuzz on fish: what it is and how to treat it

White, cotton-like fuzz on a fish is usually a water mould called Saprolegnia growing on an injury or a patch of damaged skin. Before you treat it as fungus, rule out columnaris, a fast-moving bacterial infection that also looks fuzzy and is often called "mouth fungus" even though it isn't a fungus at all.

True fungus almost never attacks a healthy fish. It follows an injury, rough handling, cold water, another disease or poor water quality. Treat the fuzz, but also fix whatever let it in.

How to identify it

	True fungus (Saprolegnia)	Columnaris (bacterial)	Epistylis (protozoan)
Looks like	Fluffy white or grey cotton wool with long strands that wave in the water; can turn brown or green as it traps dirt	Flat, greyish-white or yellowish patches, slimy rather than fluffy; ragged mouth; a pale "saddle" across the back	Small, raised white tufts, often on fins or spines, sometimes on a reddish sore
Where	On wounds, torn fins, eyes, or anywhere the skin is damaged	Mouth, lips, back near the dorsal fin, fin edges, gills	Fins, spines, skin; common on plecos and catfish spines
Speed	Spreads over days	Can spread in hours and kill within a day or two, especially in warm water	Slow
Guide	This page	Columnaris	This page

True fungus

Saprolegnia is a water mould. Strictly speaking it isn't a true fungus, but it behaves like one and responds to antifungal treatments. Its spores are in almost every tank, living on dead plants and leftover food. It attacks live fish through damaged skin and unscaled areas such as the head and gill covers.

Fungus on eggs

Eggs that turn white and fuzzy are usually dead or unfertilised eggs being eaten by water mould. The mould then spreads to healthy eggs next to them. This is very common with eggs from plecos, cichlids, killifish and tetras.

Epistylis

Epistylis is a single-celled organism that forms colonies on stalks. It feeds on bacteria in dirty water and attaches to damaged skin and fins. It is often found together with Aeromonas bacteria, which cause red sores underneath. It is easily mistaken for fungus.

When it's a problem

Early: a small tuft on a torn fin or a scrape, fish active and eating. Clean water and a mild treatment usually work.

Serious: large or several patches, fuzz on the body or head, or fuzz on the eyes or gills.

Emergency: fuzz around the mouth or a patch across the back that grows within hours, or several fish affected at once. Treat as columnaris now.

What to do right now

Look closely with a torch from the side. Long, waving strands mean fungus; flat slimy patches on the mouth or back mean columnaris.
Test the water for ammonia, nitrite, nitrate and temperature.
Do a 30–50% water change with conditioned, temperature-matched water.
Remove dead fish, dead eggs, rotting plants and leftover food. These feed the mould.
Move the affected fish to a hospital tank if you can. This protects tankmates, plants and your filter bacteria from the medicine.
For eggs: remove white, fuzzy eggs with a pipette or tweezers as soon as you see them.

What to do next

Treating true fungus

Treat for about a week, or as the label says, and keep the water clean throughout. Options, from mildest to strongest, by active ingredient:

Aquarium salt at a low dose (about 1–2 g per litre, added gradually) for a few days: helps small cases and the fish's slime coat. Not for very salt-sensitive fish or planted tanks you want to keep.
Methylene blue: gentle and widely used on fish and eggs. It stains silicone and decor, harms plants and can damage filter bacteria, so use it in a hospital tank.
Malachite green (often combined with formalin): effective against water moulds and many parasites. It is toxic at higher doses, and scaleless fish, tetras, loaches, plecos and some catfish are sensitive: use half dose for them, or choose another option. Not safe for shrimp or snails.
Potassium permanganate: a strong oxidiser used by vets and fish farms. It works, but it is easy to overdose and burn the gills. Best left to experienced keepers or a vet.
Swabbing: for a single large fish, some keepers gently net it and dab the patch with a cotton bud dipped in an antifungal solution. Do this quickly and only on bigger, hardier fish.

Always remove carbon from the filter while treating, add extra aeration (many medicines lower oxygen), don't mix medicines, and finish the full course even if the fuzz falls off early. The fish may look patchy afterwards; the skin and fins heal over the next few weeks.

Treating fungus on eggs

Remove bad eggs daily. Breeders often add methylene blue to the hatching container and keep gentle water movement over the eggs (an airstone nearby). Hatcheries also use hydrogen peroxide, but dosing for eggs is narrow and species-specific, so it's best avoided unless you have a tested protocol.

Treating Epistylis

Clean the water first: Epistylis thrives in water with lots of waste. Salt and formalin-based or "white spot and fungus" treatments are commonly used. If there are red sores underneath, the bacterial infection may also need treatment; many antibacterials require a vet prescription, depending on your country.

If it's columnaris

Antifungals won't cure it. Go to the columnaris guide, lower the temperature slightly if your fish can take it, and get advice quickly.

Products that help

Methylene blue: gentle antifungal for fish and eggs; best in a hospital tank.
Malachite green or malachite green plus formalin: stronger antifungal and antiparasitic; take care with sensitive species.
Aquarium salt: supportive treatment for mild cases.
Hospital tank with heater, sponge filter and airstone: keeps medicines away from plants, shrimp and your main filter.
Liquid test kit: finds the poor water that let the mould in.
Pipette and tweezers: for removing dead eggs.

Keep on hand: methylene blue, aquarium salt and a spare sponge filter kept running in your main tank so a hospital tank can be set up in minutes.

How to prevent it

Keep the water clean: weekly water changes and no leftover food.
Handle fish gently and as little as possible; net them into a container, not out into the air.
Remove sharp decor that tears fins.
Keep the temperature steady and right for the species.
Treat injuries and fights early. See aggression.
Remove dead eggs and dead fish quickly.
Quarantine new fish.

Why it happened

Water mould spores are everywhere, feeding on dead material. They only take hold on a fish when its skin barrier is broken by an injury, rough handling, parasites or another infection, or when stress, cold or dirty water weaken its defences. Fungus is usually the second problem, not the first.

When to get help

Get help if the fuzz spreads fast, is on the mouth or gills, returns after a full treatment, or several fish are affected. An aquatic vet can look at a scraping under a microscope and tell fungus, columnaris and Epistylis apart in minutes.

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