Cotton Wool Disease

Complete guide to identification, timeline, and treatment

What is Cotton Wool Disease?

Cotton Wool Disease is caused by the fungus Saprolegnia spp. (and sometimes the bacterium Flavobacterium columnare). It presents as white or grey fluffy growths resembling cotton wool on the fish's body, fins, or head. It is an opportunistic infection that typically strikes fish already weakened by injury, stress, or poor water quality.

Symptoms

White or grey fluffy growths on body, fins, or head
Cotton-like tufts around wounds or ulcers
Reddened or inflamed skin beneath the growth
Lethargy and loss of appetite
Fin deterioration around affected areas
Rapid breathing if gills are affected

Causes

Fungal spores (Saprolegnia) present in all aquariums, activated by stress
Wounds or injuries providing entry points for infection
Poor water quality — high organics, low oxygen
Cold water temperatures — Saprolegnia thrives in cooler water
Weakened immune system from stress, disease, or malnutrition

Cotton Wool Disease Life Cycle and Timeline

Day 1–3
Small white tufts appear on wounds, fins, or around the mouth. Fish may show mild lethargy or flashing.
Day 4–7
Growths expand rapidly. Underlying tissue becomes inflamed and ulcerated. Fish appetite declines. Secondary bacterial infections may begin.
Week 2
Large cotton-like masses cover affected areas. Tissue necrosis beneath the growth. Gills may become involved, causing respiratory distress.
Week 3+
Without treatment, systemic infection and organ failure can occur. Early treatment in the first 1–3 days gives the best outcome.

Treatment Protocol

  • Quarantine the affected fish immediately to prevent spread and reduce stress
  • Perform a 30–50% water change and improve water quality — remove excess organics
  • Raise water temperature slightly (if species-appropriate) to inhibit Saprolegnia growth
  • Treat with an antifungal medication; add aquarium salt to support recovery
  • Gently remove visible fungal growth with a clean cotton swab if accessible, then apply antifungal directly
  • If bacterial co-infection is suspected (Columnaris), add an antibiotic alongside antifungal treatment

Medication Options

  • API Pimafix (pimaricin) — antifungal treatment effective against Saprolegnia; gentle and reef-safe
  • Methylene Blue — antifungal and antibacterial; effective for mild cases and egg fungus
  • Aquarium salt (1–3 tsp per gallon) — inhibits fungal growth and reduces osmotic stress
  • Seachem ParaGuard — broad-spectrum treatment effective against fungal and bacterial infections
  • Kanaplex (kanamycin) — add if bacterial Columnaris co-infection is confirmed or suspected
  • Hydrogen peroxide dip (brief, supervised) — spot treatment for localised fungal growths on fish
Important

Cotton Wool Disease is opportunistic — healthy fish in clean water rarely develop it. Always address the underlying cause (injury, stress, poor water quality) alongside treatment. Saprolegnia can also infect fish eggs; treat affected eggs promptly with Methylene Blue to prevent spread.

Important Disclaimer

The information provided in this guide is for educational purposes only. Always consult with a qualified aquatic veterinarian or fish health specialist before administering any treatments. Proceed with treatments at your own risk. We are not responsible for any adverse outcomes.