Cotton-Wool Fungus on Fish: Diagnosis and Treatment
White fuzzy patches on a fish usually signal true fungus taking hold on damaged skin, or a bacterial look-alike. This guide helps you tell them apart, correct the water problem behind it, treat safely in a hospital tank, and know when the case needs a vet.
Quick answer
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.
White fuzzy patches on a fish usually signal true fungus taking hold on damaged skin, or a bacterial look-alike.
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.
Quick FAQs
Will fungus spread to my other fish? True fungus is opportunistic and rarely jumps to healthy, unbroken skin, but the water conditions that caused it affect everyone, so treat the tank environment as well as the fish.
Can I just add medication to my main tank? It is better not to. Many anti-fungals kill your filter bacteria, triggering an ammonia spike that makes things worse. A hospital tank keeps treatment contained.
How long until it clears? Mild fungus often visibly shrinks within 3–5 days of correct treatment and clean water. If there is no improvement in a week, reassess the diagnosis with a vet.
Is the white stuff on my fish's mouth always fungus? No. Mouth 'fungus' is frequently Columnaris, a bacterium, which spreads fast and needs different treatment — one more reason to get an uncertain case checked.