Mouth fungus is misnamed - the white fluffy patches around the mouth are actually a bacterial infection (Flexibacter columnaris), not a fungus. Common in stressed freshwater fish, especially livebearers and bettas. Highly contagious + can kill an entire tank in 24-72 hours if untreated.
Aggressive immediate treatment is required. Dose Kanaplex + Furan-2 combo in the tank itself (not hospital - too contagious to delay). Daily 25% water changes + redose. Lower temperature to 75-78F (Flexibacter slows at lower temps). Treat 7-10 days. Add aquarium salt at 1 tbsp per 5 gallons.
Less common in marine. When it occurs, usually post-injury or post-stress. Move to hospital tank, dose Furan-2 + Kanaplex 10 days. Address underlying cause (parameter swing, aggression, poor water quality).
Stable parameters + low nitrate. Quarantine new fish 4-6 weeks. Avoid mixing aggressive species. Clean filtration. Stress-prone fish (bettas, fancy goldfish, livebearers) benefit from preventive aquarium salt at 1 tbsp per 10 gallons as standard.
Estimated cost: $30-50.
Untreated, columnaris bacteria can kill fish within 24-72 hours. Acute cases progress so fast that fish look fine in the morning and dead by evening. Treat immediately at first sign of white mouth patches.
Extremely. Flexibacter columnaris spreads through water and infects any fish with compromised mucus coat. Treat the entire tank, not just the visible-symptom fish.
The single most important disease-prevention step: a 4-6 week quarantine of every new fish before adding to your display. See the complete quarantine protocol.