How to Treat Saprolegnia (Water Mould) on Fish
White cottony tufts clinging to your fish are a sign that demands immediate action. The need to treat saprolegnia water mould on fish arises when Saprolegnia or related oomycete organisms exploit wounds, weakened immune systems, or poor water quality. Left unchecked, these fungal-like pathogens consume living tissue rapidly. Gensou Aquascaping at 5 Everton Park, Singapore has dealt with saprolegnia outbreaks across countless client tanks, and this guide shares the practical protocols that work.
What Saprolegnia Actually Is
Saprolegnia belongs to the oomycetes — water moulds that look like fungi but are genetically closer to brown algae. They thrive in freshwater at temperatures between 15-25°C, though Singapore’s warmer water at 28-30°C slows but does not eliminate them. Spores are present in virtually every aquarium. They become problematic only when a fish’s slime coat is compromised by injury, stress, or ammonia exposure. The cotton-like growths are masses of hyphae that penetrate tissue and feed on both dead and living cells.
Recognising the Symptoms
Early signs include small white or grey patches near wounds, fin edges, or around the mouth. These patches grow into fluffy, cotton-ball masses within 24-48 hours if untreated. Affected fish may become lethargic, lose colour, and stop eating. Gill infections cause rapid breathing and gasping at the surface. Do not confuse saprolegnia with columnaris (Flavobacterium columnare), which produces flatter, greyish patches with a slimy rather than cottony texture — treatment differs significantly.
Salt Bath Treatment
For mild cases, a salt bath is the simplest first response. Dissolve 3-5 grams of aquarium salt per litre in a separate container of tank water. Bathe the affected fish for 15-30 minutes while observing closely for signs of distress. Salt disrupts the osmotic balance of Saprolegnia hyphae, causing them to release from tissue. Repeat daily for up to five days. Be cautious with salt-sensitive species like corydoras, loaches, and most tetras — reduce the concentration to 1-2 g/L and shorten the bath to 10 minutes.
Medication Protocols
Methylene blue at 2 mg per litre is a reliable broad-spectrum treatment for saprolegnia in a hospital tank. It stains silicone and kills nitrifying bacteria, so never dose in your display aquarium. For display tanks, malachite green and formaldehyde combinations such as Proform-C or NT Labs Anti-Fungus and Finrot work well at half-dose if invertebrates are present. Hydrogen peroxide applied directly to the mould with a syringe (3% solution, undiluted, applied for a few seconds) is effective for localised infections on larger, calm fish. Always ensure good aeration during treatment since these medications reduce dissolved oxygen.
Optimising Water Conditions
Saprolegnia is an opportunist — clean water is your strongest defence. Ammonia and nitrite must read zero; nitrate should stay below 20 ppm. In Singapore, PUB tap water is soft with a GH of 2-4, which offers little mineral buffering. Adding a small amount of calcium and magnesium via remineralising salts improves fish health and slime coat integrity. Increase water change frequency to 30% every two days during active infection. A UV steriliser rated for your tank volume kills free-floating spores and reduces reinfection pressure dramatically.
Addressing the Root Cause
Treating the mould without fixing the underlying trigger guarantees recurrence. Check for sharp hardscape edges that could abrade fish. Remove aggressive tankmates causing fin damage. Verify your heater is functioning — temperature drops overnight in air-conditioned rooms can stress tropical species and open the door to infection. Overcrowding raises cortisol levels and suppresses immunity, so stock conservatively. A good rule is 1 cm of adult fish per 2 litres of actual water volume.
Recovery and Aftercare
Once the cotton-like growth disappears, keep the fish in the hospital tank for another 5-7 days to confirm no relapse. Feed vitamin-enriched foods — soaking pellets in garlic guard or a vitamin supplement accelerates immune recovery. Indian almond leaves release tannins that have mild antifungal properties and help restore the slime coat. Reintroduce the fish to the display tank only after confirming water parameters are stable and no other tankmates show symptoms.
Preventing Future Outbreaks
Quarantine all new fish for at least 14 days before adding them to your main tank. Net fish gently to avoid scale damage during transfers. Remove uneaten food within a few hours to limit organic waste that feeds saprophytic organisms. If you frequently battle saprolegnia, consider running a small UV clarifier permanently — units suitable for 50-100 litre tanks cost around $30-50 on Shopee or Lazada and draw minimal power. Prevention is always simpler than cure.
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emilynakatani
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5 Everton Park #01-34B, Singapore 080005 · Open daily 11am – 8pm
