Quick referenceSaprolegnia parasitica is the cotton-wool fungal infection of freshwater fish + eggs. Treat with methylene blue, Pimafix, or formalin baths; prevent with stable temperatures.
Symptoms
Cotton-like growth
Frayed/torn fins
Color fade
Refusing food
White wispy patches on eggs
Cause
Saprolegnia parasitica fungus is opportunistic - colonizes injured/stressed fish + dead eggs. Triggers: cold water, recent injury, poor water quality, scale damage from rough nets.
Treatment options
Methylene blue. Effective for fish + egg fungus. Bath at 3 ppm for 30 min, or full-tank treatment in QT. Stains silicone + plastic.
Pimafix (Melafix companion). Botanical antifungal. Mild but works on early-stage fungus.
Formalin bath. Aggressive. 30-min bath in 1 tsp formalin per gallon. Use only in advanced cases.
Salt bath. Aquarium salt 1 tsp/gal for 7-10 days. Antifungal effect, fish-tolerant.
Improve water quality. Fungus rarely strikes healthy fish in clean tanks.
Always treat in a separate quarantine or hospital tank. Most medications are toxic to coral, invertebrates, and live rock biology. Consult an aquatic veterinarian for valuable fish.
Prevention
Maintain stable temperature (no cold drops). Use soft nets to avoid scale damage. Don't overcrowd. Quarantine new fish. Remove dead eggs from breeding tanks immediately.
Fatality + outcome
Moderate - secondary fungal infection on injuries or stressed fish kills 30-50% if untreated. Healthy fish in clean water clear it.