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bacterial fungal

Fungal infection (Saprolegnia / Achyla)

White, grey, or brownish cottony or woolly tufts growing on the skin, fins, or around the eyes. Saprolegnia and Achyla are water molds (oomycetes) that almost always infect already damaged tissue. They are secondary invaders following injury, bacteria, or parasites.

Medium prioritypattern match not diagnosis1 source note

Do first

  • Identify and treat the primary cause (injury, ich, bacteria) fungal infections are nearly always secondary.
  • Test and correct water quality ammonia and cool temperatures dramatically increase fungal risk.
  • Separate affected fish to a hospital tank if possible to prevent spread.
  • Begin antifungal treatment.

Escalate if

  • Fungal growth spreading to the gills or eye (especially if the eye appears to be dissolving).
  • No improvement after 5–7 days of correct antifungal treatment.
  • Fish developing additional systemic symptoms (lethargy, not eating, color loss) suggesting a concurrent bacterial infection.

Water clues

These readings can push this pattern higher or lower in the triage result.

ammonia above zero+4

Ammonia-damaged skin is highly susceptible to Saprolegnia colonization fixing water quality is essential.

temp below species min+3

Saprolegnia thrives in cooler water and cold-stressed fish have reduced immunity.

Care protocol

Follow only the steps that fit your species, tank inhabitants, and medication label.

Confirming Saprolegnia

  1. Saprolegnia forms soft, fluffy white to grey-brown cotton-wool tufts that move gently in the water current.
  2. Unlike Columnaris (which has a drier, flat appearance), Saprolegnia looks clearly fuzzy and three-dimensional.
  3. Often begins at a wound, fin tear, or site of previous disease.
  4. Eggs in spawning tanks are also very commonly affected by Saprolegnia.
Cautions
  • True fungal infections are common but so is confusion with Columnaris they require different treatments.
  • Do not treat with antifungals if Columnaris (bacterial) is more consistent with the presentation.

Treatment protocol

  1. Treat with methylene blue, malachite green, or a commercial antifungal product designed for aquarium fish.
  2. Methylene blue baths (short duration) are particularly effective follow dosage instructions carefully.
  3. Improve water temperature to the species' optimal range warmth inhibits Saprolegnia growth.
  4. Perform regular water changes to reduce fungal spore load in the water.
  5. Address the underlying cause (repair aggression, treat bacterial infection, correct water quality).
  6. Salt at 1–2 g/L as a low-level background addition can reduce fungal growth in tolerant species.
Cautions
  • Malachite green is toxic to scaleless fish at full doses halve doses for loaches, catfish.
  • Malachite green should not be used in tanks with food fish intended for human consumption.
  • Do not use methylene blue in tanks with biological filtration it kills beneficial bacteria.

Source notes

References and context notes used for this triage entry.

Saprolegnia (Water Mold) Infections UF/IFASExtension reference for fungal diseases of fish.
Run Symptom CheckerCompare this pattern against water readings, affected count, and recent tank context.Ask AdvisorUse TankFlare Advisor to review this against your saved tank and recent logs.

Related patterns

Similar symptoms or risk factors worth comparing before treatment.

High priorityColumnaris (cotton mouth / saddleback disease)

Pale grey-white patches on the mouth, body, or dorsal surface often with a saddle-shaped lesion behind the dorsal fin. Caused by Flavobacterium columnare, a gram-negative bacterium. Can progress very rapidly, especially at warmer temperatures.

Medium priorityFin rot or tail damage

Progressive erosion or fraying of fins and tail tissue, often starting at the edges. Can be bacterial (Aeromonas, Pseudomonas, Flexibacter) or fungal, and is strongly associated with water quality issues and physical injury.

UrgentBacterial septicemia (hemorrhagic septicemia)

Red streaking, hemorrhaging under the skin, and systemic illness caused by gram-negative bacteria (most commonly Aeromonas hydrophila) entering the bloodstream. Can follow injury, fin rot, poor water quality, or be primary in severely stressed fish. A systemic emergency.

Medium priorityChronic stress syndrome and immune suppression

A fish that is persistently stressed over weeks or months will develop a suppressed immune system, making it vulnerable to infections that healthy fish would fight off. Chronic stress is itself a disease state that produces predictable signs and predisposes fish to virtually every other condition in this catalog. Identifying and removing the stressor is the only effective treatment.