Quick Answer
Saprolegnia, commonly called true fungus or water mold, is an opportunistic oomycete (Stramenopiles) that colonizes dead or damaged tissue on fish, producing fluffy white or grey cotton-wool growths. This is a medium-severity condition because the oomycete itself rarely kills healthy fish, but it signals underlying tissue damage, bacterial infection, or poor water quality that can become lethal if untreated. Treat with methylene blue at 3 mg per liter as a 30-minute bath or 1 mg per liter for prolonged pond treatment, or use acriflavine-based products such as API Fungus Cure according to label instructions for 7–10 days. Every day of delay allows the oomycete hyphae to penetrate deeper into compromised tissue, and secondary bacterial infections such as Aeromonas or Pseudomonas often colonize the same wounds simultaneously.
Treatment Options Compared
| Method | Effectiveness | Cost | Time to Resolve | Fish/Pond Stress Level |
|---|---|---|---|---|
| Methylene blue bath (3 mg/L, 30 min) | High – directly kills oomycete hyphae on contact | Low ($8–15) | 3–7 days (repeat baths every 48 hours) | Low–Moderate – brief handling stress |
| Acriflavine (API Fungus Cure) tank treatment | Moderate–High – suppresses oomycete and bacterial biofilms | Low ($8–12) | 7–10 days | Low – gentle on fish, safe for biofilter |
| Malachite green (alone) | High – potent antifungal but toxic to invertebrates | Low ($5–10) | 5–7 days | Moderate – stains silicone, toxic to shrimp and snails |
| Hydrogen peroxide dip (3%, 1–2 mL/L, 5 min) | Moderate – oxidizes oomycete tissue directly | Very Low ($2–3) | Immediate for visible tufts | High – risky if concentration or timing is off |
| Salt (1–3 ppt) + pristine water | Low – inhibits oomycete growth marginally | Very Low ($3–5) | 1–2 weeks (often insufficient alone) | Very Low – supportive only |
Troubleshooting Guide
1 Fungal tufts cleared with methylene blue but returned within a week.
The underlying tissue damage or bacterial infection was not treated. Fungus is always secondary. Treat the primary wound with a broad-spectrum antibiotic such as kanamycin, maintain pristine water quality, and only then address the fungal overgrowth.
2 Fish developed red gills and gasping after methylene blue bath.
Methylene blue at high concentrations can damage gill tissue. Reduce the bath concentration to 1 mg per liter and limit exposure to 15 minutes. Increase aeration immediately and perform a water change if gasping persists.
3 Fungus appeared on multiple fish simultaneously after a cold snap.
Temperature drop below 18°C/64°F suppresses immune function and accelerates fungal growth. Raise temperature gradually to 22–24°C/72–75°F if species-appropriate, perform a large water change, and treat with methylene blue or acriflavine. Check for underlying bacterial infections that opened the tissue.
4 Hydrogen peroxide dip caused the fish to roll over and lose equilibrium.
The peroxide concentration was too high or the dip duration exceeded safe limits. Immediately return the fish to clean, well-aerated tank water. Never exceed 2 ml per liter for 5 minutes. Methylene blue baths are safer for most hobbyists.
Frequently Asked Questions
What does true fungus look like?
How is fungus different from Columnaris?
Can fungus grow on healthy tissue?
Why is methylene blue used?
Is fungus contagious?
Can I use hydrogen peroxide for fungus?
How do I prevent fungus on eggs?
Does fungus need light to grow?
Can salt cure fungus?
Will fungus clear on its own after a water change?
Why does true fungus outbreak more often in cold water or after temperature drops?
Does quarantining new fish prevent true fungus outbreaks in the main tank?
Glossary of Terms
- Saprolegnia
- A genus of oomycete water molds commonly called true fungus; it colonizes dead or damaged tissue on fish, producing fluffy white or grey cotton-wool growths. Taxonomically placed in Kingdom Stramenopiles (Chromista), not Kingdom Fungi.
- Oomycete
- A group of fungus-like organisms in Kingdom Stramenopiles including Saprolegnia; they have cellulose cell walls, diploid nuclei, and produce motile biflagellate zoospores, making them more closely related to brown algae and diatoms than to true fungi.
- Methylene blue
- A thiazine dye with strong antifungal and mild antibacterial properties; used as a bath treatment for external oomycete infections and to improve oxygen transport in fish with gill damage.
- Acriflavine
- A topical antiseptic and antifungal agent used in aquarium products such as API Fungus Cure; effective against Saprolegnia and some bacterial biofilms.
- Zoospore
- The motile, biflagellated reproductive spore of oomycetes such as Saprolegnia; zoospores swim through water and attach to compromised tissue to initiate infection.
Scientific References
- Saprolegniasis in Freshwater Fish: Diagnosis and Treatment
- Oomycete Infections in Ornamental Aquarium Fish
- Methylene Blue and Acriflavine Protocols for External Oomycete Infections
- Environmental Factors Influencing Saprolegnia Outbreaks in Aquaculture