Saprolegnia (Water Mold) Treatment Guide

Saprolegnia is an oomycete (water mold, not a true fungus) causing cottony tufts on injured fish and eggs; almost always secondary to stress or poor water quality.

Overview

Saprolegnia is a genus of water molds (oomycetes, phylum Oomycota), which are heterokonts rather than true fungi despite their cotton-like appearance. The two most relevant species in aquaria are Saprolegnia parasitica and Saprolegnia ferax. They produce characteristic white or grey cotton-like patches on fish skin, fins, gills, and especially on dead or unfertilised eggs in breeding tanks. Infection is almost always secondary to injury, stress, or poor water quality.

Symptoms

White to grey cottony tufts on body, fins or gills
Grey film or filaments on damaged areas
Cottony patches on eggs in breeding tanks
Lethargy and loss of appetite as infection spreads
Reduced respiration when gills are affected
Often accompanied by secondary bacterial infection

Causes

Saprolegnia spores are ubiquitous in freshwater systems. They opportunistically colonize dead tissue or weakened areas after physical injury, abrupt temperature drops, prolonged exposure to chlorinated water, ammonia or nitrite spikes, or after prolonged transport. In breeding tanks, dead and unfertilised eggs become Saprolegnia reservoirs that can then infect viable eggs.

Diagnosis

Diagnosis is visual: cottony grey-white tufts on damaged tissue. Differentiate from columnaris (Flavobacterium columnare), whose patches are flatter, yellow-tinged at the edges and spread across healthy skin, and from epistylis (a ciliated protozoan) whose tufts are smaller and stalked. Saprolegnia almost always centres on a pre-existing wound.

Treatment

Step 1: Quarantine

Move affected fish to a quarantine tank if possible. Identify and correct the underlying stressor — without addressing the root cause (injury, poor water, temperature) re-infection is almost certain. Remove activated carbon during medication.

Step 2: Medication

Aquarium salt (sodium chloride) at 1 teaspoon per 4 L in a quarantine tank for 5-7 days; not for salt-sensitive species such as Corydoras or scaleless fish at this concentration.
Methylene blue bath at 3 ppm for 30 minutes for early external infection; effective and gentle on most species.
Pimafix (Melaleuca-derived) per package for 7 days for mild cases.
Formalin bath at 25 ppm for 60 minutes for severe cases; use only under strong aeration.
For infected eggs: methylene blue 2-3 ppm in the breeding tank from spawning to hatching.

Step 3: Recovery

After 5-7 days of treatment, perform a 30-50% water change, restore activated carbon, and gradually return fish to the main tank once tissue regrowth is visible. Maintain stable parameters and a varied diet during recovery to support immunity.

Prevention

Minimize physical injury during catching and transport
Maintain stable temperature and good water quality
Remove dead or unfertilised eggs immediately in breeding tanks
Quarantine new fish for 2-4 weeks
Provide a balanced diet to support immune response