Cotton-Wool Fungus on Fish: Diagnosis and Treatment

Quick answer

Cotton-Wool Fungus on Fish: Diagnosis and Treatment

A white, cotton-wool or fuzzy patch anchored to one spot on the body or fins is usually a true fungus (Saprolegnia or Achlya), and it almost always follows an injury, a bite, or poor water quality. Move the fish to a hospital tank, fix the underlying water problem, and treat with an anti-fungal or a salt bath. Fast-spreading grey-white slime that coats the fish is more likely a bacterial infection (Columnaris) and needs different treatment.

Decide in 60 seconds

Is it really fungus?

True fungus grows as a distinct fluffy tuft you could almost pinch, usually greyish-white, sprouting from a wound, an old ulcer, or the mouth. It stays anchored to one area at first. It thrives on tissue that is already damaged, so ask what injured the fish: sharp decor, a fin nip, a rough net, or ammonia burn from a spiking tank.

True fungus looks like a distinct fuzzy white tuft anchored to one spot.

Fix the water first

Medication without husbandry will fail. Test ammonia, nitrite and nitrate. Ammonia and nitrite should read zero; anything above that is burning your fish and feeding the outbreak. Do a 25–40% water change with dechlorinated, temperature-matched water, gently vacuum the substrate, and check that your filter and heater work. Overfeeding and uneaten food rotting in the gravel are common hidden triggers.

Treating in a hospital tank

Set up a separate bare-bottom tank with an air stone and heater so you can medicate without destroying the beneficial bacteria in your main filter.

Treat in a separate hospital tank so medication does not harm your filter bacteria.

For mild, localised fungus on a hardy fish, aquarium salt (a plain sodium chloride, not table salt with iodine or anti-caking agents) at a measured low dose, or a proprietary anti-fungal such as one containing methylene blue or malachite green, is the usual route. Follow the product instructions exactly and finish the full course. Never combine multiple medications hoping for a stronger effect , many are toxic together.

Salt baths for stubborn cases

A short salt dip can help debride fungus on tolerant species, but salt tolerance varies enormously , some catfish, loaches and soft-water species react badly. Confirm your species tolerates salt before you start, keep baths brief and supervised, and return the fish to clean water at the first sign of distress.

Preventing the next outbreak

Most fungus is a symptom of something else. Keep stable, clean water; quarantine new arrivals; remove sharp decor; avoid overcrowding; and feed sparingly. A well-run tank rarely sees fungus at all.

Living context by region

This guide is written for multi-region English readers (US/UK/AU/NZ/CA/EU and more). Where climate, products, or vet access differ, prefer local veterinary advice and local product labels. Use metric first; keep °F/lb in mind for North American homes.

When to call your vet

• Signs are getting worse over hours, not better

• Pain, persistent vomiting/diarrhea, or neurological signs

• You needed force or guesswork to keep going at home

Common mistakes

• Waiting because the animal 'still looks okay' while prey species hide pain

• Changing five variables at once so you cannot tell what helped

• Copying drug doses from social media

• Skipping species-appropriate husbandry while buying more gadgets

FAQ