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Act within daysOomycete (water mould)Disease with an established agent

Water mould (Saprolegnia)

Saprolegnia and its relatives are ubiquitous in fresh water and are primarily saprotrophic: they consume dead organic matter. On a healthy fish with an intact mucus layer they gain no foothold. Their appearance on a living fish is therefore a signal that the tissue was already compromised — by injury, by handling damage, by a parasite, by chilling, or by an existing bacterial lesion. Treating the mould while leaving the underlying cause in place produces exactly the outcome you would expect. The one genuinely primary presentation is on eggs, where mould readily colonises infertile eggs and then spreads to viable ones.

Short answer

The cotton-wool growth on a damaged fish is a water mould, not a true fungus — oomycetes belong to a different kingdom entirely. It is almost always a secondary coloniser of tissue that something else damaged first, which means the useful question is not how to treat the growth but what injured the fish.

What you can and cannot establish yourself

Suggestive by eye, not conclusive

You can see it, but several conditions look like this. Work through the alternatives below before deciding what you are treating.

Cause
A water mould that colonises damaged or dead tissue — Saprolegnia spp. and related oomycetes
Spread between fish
Rarely spreads directly
Urgency
Act within days
Affects
fish
Confidence
moderate confidence

Signs

  • White or greyish cotton-wool-like growth, raised above the skin surface
  • Filaments visibly streaming in the water current
  • Growth centred on an existing wound, injury or damaged area
  • Sometimes a brownish or greenish tint where debris or algae are trapped in the filaments
  • On eggs, a fuzzy growth spreading outward from dead eggs to adjacent ones

Do this first: the environmental response

Water quality is the cause, the contributing factor or the deciding variable in the great majority of aquarium health problems. These steps come before any treatment decision, and for several conditions they are the entire treatment.

  1. Find and remove the underlying cause. This is the actual treatment: an injury source, a parasite, a chill, a bully, or poor water quality.
  2. Test ammonia and nitrite, and improve water quality — clean water is what allows the wound beneath the growth to heal.
  3. Check the temperature is correct for the species. Chilling is a very common precipitant, particularly in unheated or seasonally cold tanks.
  4. Remove sharp decor and reduce handling.
  5. For eggs, remove infertile eggs promptly and maintain good water movement across the spawn.

What else it could be

Every entry names the observation that separates it from this record. Work through these before deciding what you are treating — the most expensive mistake in fish health is confident treatment of the wrong thing.

Non-disease explanations — check these first

  • Excess mucus over an irritated area

    Husbandry causecommon

    How to tell it apart: Mucus is a smooth film that follows the body contour. Mould is a three-dimensional growth with visible filaments.

Other conditions

  • Columnaris

    Named conditionvery common

    How to tell it apart: Columnaris lesions are flatter, greyer and often slightly sunken, with defined margins, and progress far faster. Water mould stands proud of the skin and its filaments move in the current. The mouth presentation is the one most often confused, and it is usually bacterial rather than mould — which is why 'mouth fungus' is a misleading name.

  • Epistylis

    Named conditioncommon

    How to tell it apart: Epistylis forms smaller, more discrete tufts rather than a continuous cotton-wool mass. Both colonise damaged tissue, and both indicate that something else came first.

  • Whitespot

    Named conditionoccasional

    How to tell it apart: Whitespot produces small discrete specks distributed over the body. Mould produces a localised mass at one site.

What makes it more likely

Each entry states the mechanism, not just the association — an association without a mechanism is not something you can act on.

  • Any existing skin injury. The mucus layer is the barrier. Where it is broken, the organism has access to tissue it can colonise.
  • Chilling or a temperature below the species' range. Suppresses immune function and slows wound healing while the mould remains active, which is why it is classically a coldwater and winter problem.
  • Poor water quality and high organic load. Increases the spore load in the water and damages epithelium at the same time.
  • Handling and netting. Physically removes mucus and damages skin. A great many cases follow a move.
  • Infertile eggs left in a spawn. Dead eggs are ideal substrate, and mould spreads from them to viable eggs adjacent.

Treatment categories

AquariumHQ names classes of response and never doses. Product concentrations differ between markets and formulations, several actives are temperature- and pH-dependent, and the invertebrate and plant tolerances that decide whether the rest of the tank survives are not printed on the label. Where a category puts a chemical in the water, what it can damage is stated with it.

Correct the husbandry

Correcting the cause of the original damage and providing clean, correctly warmed water is what resolves most cases. The growth recedes as the tissue beneath it heals.

Antifungal

Adds a chemical to the water

Products marketed against fish fungus target the water mould and are appropriate for established growth, particularly where a large area is involved or the fish is deteriorating. They do not address whatever damaged the fish first.

What it can damage: Antifungal products for aquarium use are commonly malachite-green or methylene-blue based. Both are hazardous to invertebrates, malachite green damages some plants and stains silicone and decor, and both reduce the effectiveness of biological filtration in some formulations.

Do not use

  • Tanks containing ornamental invertebrates
  • Use as a substitute for identifying the underlying injury

Salt

Adds a chemical to the water

Salt is a traditional supporting measure for water mould in species that tolerate it, and it also reduces the osmotic burden on a fish with damaged skin.

What it can damage: Many aquarium plants tolerate salt poorly, ornamental shrimp and snails are sensitive, and several soft-water fish — including a number of catfish and loaches — do not tolerate it well. AquariumHQ gives no concentration: the appropriate level differs by species and by purpose, and the figures circulating in the hobby are frequently transcribed from marine or aquaculture contexts.

Do not use

  • Planted tanks
  • Tanks containing shrimp or snails
  • Species with poor salt tolerance, including many soft-water catfish and loaches

Veterinary referral

Appropriate where a large area is affected, where the underlying cause cannot be identified, or where the fish is deteriorating despite correction.

Isolation and quarantine

  • Isolation is useful for an affected individual — it allows excellent water quality, removes competition and prevents further physical damage while the wound heals.
  • It is not needed to protect other fish, because the organism is already present in every freshwater system and healthy fish are not susceptible.

When a vet is the right call

  • Growth covering a substantial area or involving the eyes or gills
  • No identifiable underlying cause
  • Deterioration despite correction of water quality and temperature

AquariumHQ describes what is commonly reported and what husbandry correction is usually indicated. It does not diagnose and is not a substitute for an aquatic veterinarian. Many treatments are given to healthy fish suffering from a water-quality problem, which makes the outcome worse rather than better.

Prevention

  • Handle fish as little as possible, and use containers rather than nets where a fish must be moved.
  • Keep temperature within the species' range and stable — chilling is the classic precipitant.
  • Remove sharp decor and address aggression that causes injuries.
  • Remove infertile eggs from spawns promptly.
  • Maintain water quality, which both reduces spore load and speeds healing.

What AquariumHQ does not know

Published rather than hidden. A health page that lists no limitations has not had its limitations looked for.

  • Species-level identification within the water moulds is not something AquariumHQ holds, and it is rarely established in aquarium cases.
  • Whether some presentations are genuinely primary rather than secondary is debated; AquariumHQ treats the secondary pattern as the working assumption because that is what the veterinary sources describe.

Evidence

Each claim is shown with what kind of support stands behind it and how strongly we hold it. Those are separate axes: a measurement held tentatively and an editorial position held firmly are different things.

Observed / measuredhigh confidence

Saprolegnia and related oomycetes — water moulds, which are not true fungi and sit in a separate kingdom

  • MSD/Merck Veterinary Manual — fish chapters — Fish — saprolegniasis
  • Noga, E.J. — Fish Disease: Diagnosis and Treatment (2nd edn, 2010) — Oomycete infections

Last verified 2026-08-11

Sourced recommendationhigh confidence

Infection of live fish is characteristically secondary to existing tissue damage, immunosuppression or chilling rather than primary

  • Noga, E.J. — Fish Disease: Diagnosis and Treatment (2nd edn, 2010) — Oomycete infections — pathogenesis
  • UF/IFAS Extension — fish health and aquaculture publications — Ornamental fish health — fungal and oomycete disease

Last verified 2026-08-11

Observed / measuredhigh confidence

Commonly called 'fungus' in aquarium usage; oomycetes are not fungi, which is why treatment reasoning drawn from mycology does not transfer

  • Noga, E.J. — Fish Disease: Diagnosis and Treatment (2nd edn, 2010) — Oomycete infections — taxonomy
  • MSD/Merck Veterinary Manual — fish chapters — Fish — saprolegniasis

Last verified 2026-08-11