Saprolegnia (fungus) in discus: step-by-step white cotton-wool treatment

In short

• Cause: Saprolegnia spp. and Achlya — water fungi (oomycetes) present naturally.
• Appearance: white cotton-wool / mold filaments, bright, fluffy — not flat patches (see Columnaris).
• Typical on: old wounds, non-viable eggs, weakened fish or fish with other diseases.
• It is NOT an aggressive opportunist — it attacks only dead or damaged tissue.
• Main treatment: Methylene Blue 2 mg/L OR JBL Fungol OR salt 3 g/L (hospital tank).
• Crucial for breeding: on eggs — Methylene Blue in the breeding tank.
• Prevention: clean water + decor with no sharp edges + a stable temperature of 29°C (84°F).

What you'll learn from this guide

Saprolegnia in discus (Symphysodon) is the most common fungal infection in fishkeeping — it appears as white cotton-wool or mold filaments on the body, fins and especially on non-viable eggs in the breeding tank. Unlike Columnaris (an aggressive bacterium on living tissue), Saprolegnia attacks only dead, wounded or weakened tissue — it's an indicator that something is wrong in the environment or on the fish. This guide shows you how to clearly tell Saprolegnia from Columnaris (most important), how to treat with Methylene Blue, JBL Fungol or salt, how to protect breeding eggs, and how to identify the primary cause (wound? disease? bad water?) to prevent a relapse.

What Saprolegnia is — biology

Key idea: Saprolegnia is not an "aggressive" disease — it's a symptom that the fish is weakened or wounded. Its appearance is a sign that you must look for the primary cause.

Triggers — what activates Saprolegnia

⚠️ Saprolegnia rarely appears on its own — the primary cause (wound, disease, bad water) must ALWAYS be identified.

Symptoms — visual diagnosis

On the body / fins

• ✅ Bright white filaments, like cotton wool or mold
• ✅ Fluffy, sticky, 3D appearance (it moves in the current)
• ✅ Edges without inflammation (NOT red like Columnaris)
• ✅ Located on old wounds or weak areas
• ✅ The fish is still active initially — Saprolegnia is not acutely lethal
• ✅ Slow spread (days-weeks)

On eggs (breeding tank)

• ✅ White or opaque (non-viable) eggs
• ✅ White filaments around them that spread to neighboring eggs
• ✅ The parents ignore or "clean" the affected area
• ✅ Risk: viable eggs contaminated by Saprolegnia from the dead ones

Advanced stage (rare in discus)

• ❌ Filaments cover large areas (>20% of the body)
• ❌ Lethargic fish, food refusal
• ❌ Secondary bacterial infections (Aeromonas on lesions)
• ❌ Mortality in very weakened fish

Saprolegnia vs Columnaris — THE CRITICAL DIFFERENTIATION

This is the most important diagnosis — antifungal treatment does NOT work on Columnaris, and lost time = mortality.

🎯 Quick test: got a white patch on a fish? Check:

• Bright fluffy 3D cotton wool → Saprolegnia (antifungal)
• Flat yellowish on the mouth → Columnaris (antibiotic, EMERGENCY)

When in doubt, treat it as Columnaris — it's more dangerous if you get it wrong the other way.

Treatment — step by step

Step 1 — Identify and treat the PRIMARY CAUSE

Saprolegnia is a symptom, not a cause. Before the antifungal, identify:

Step 2 — Antifungal treatment

Option A: Methylene Blue ⭐ TOP RECOMMENDATION (safety)

Procedure (hospital tank):

1. Set up a hospital tank of 40-80L (11-21 US gal) with a mature sponge filter
2. 50% water from the main tank + 50% fresh dechlorinated at 29°C (84°F)
3. Remove the activated carbon (it absorbs it completely)
4. Add Methylene Blue 2 mg/L (~10 ml of 1% solution for 50L)
5. Maximum aeration
6. 50% change / day + re-dose
7. Total 5-7 days or until the filaments disappear
8. Post-treatment: new activated carbon + a 50% change to remove the color

Option B: JBL Fungol Plus 250

Option C: eSHa 2000

• Also contains antifungals — broad spectrum
• approx. €9–15 / 20 ml (RO estimate)
• 5 ml / 50L (13 US gal), 5 days
• Useful when you also suspect a secondary bacterial infection

Option D: Sera Mycopur (fungus-specific)

• approx. €11 / 50 ml (RO estimate)
• Per the manufacturer
• Antifungal + bacteriostatic

Option E: Non-iodized salt (a natural alternative)

Salt procedure:

1. Dissolve 3 g of salt per liter in warm water
2. Add gradually over 2 hours to the hospital tank
3. 25% change / day with SALTED water at the same concentration
4. 7 days maximum, then 50% changes to remove the salt

🧮 For the exact dose on the true volume (minus substrate / decor), use the treatment calculator.

Step 3 — Treating Saprolegnia on EGGS (breeding)

Very common with discus breeders — Methylene Blue is the global standard.

Breeding-tank protocol

Methylene Blue does NOT harm viable eggs — it stains the water blue, but it's safe. It clearly distinguishes white eggs (affected) from pinkish-translucent ones (viable).

Step 4 — Recovery support (after treatment)

Recovery time:

• Filaments disappear: 3-5 days after treatment
• Healing of primary wounds: 1-3 weeks
• Full slime-coat regeneration: 2-4 weeks
• Relapses possible if the primary cause persists

Common mistakes

❌ Confusing it with Columnaris → you treat something bacterial with an antifungal → mortality. When in doubt, treat as a BACTERIUM (more dangerous if you get it wrong the other way).

❌ Treating without identifying the primary cause → relapses for weeks. Saprolegnia is a SYMPTOM.

❌ Salt in a planted main tank → many plants die at >1 g/L. Use salt only in the hospital tank.

❌ Methylene Blue with sensitive plants long-term → the blue reduces photosynthesis, plants suffer. Use a hospital tank for long treatments.

❌ Activated carbon in the filter → it absorbs Methylene Blue / JBL Fungol within 12h. Always remove it.

❌ Stressing the weakened fish with big changes → 25% changes are enough, not 80%.

❌ Fish with a destroyed slime coat reintroduced without recovery → they get Saprolegnia again. Wait 2 weeks with a vitamin supplement + catappa leaves.

❌ Affected eggs left untreated → they contaminate the neighboring viable eggs. Methylene Blue from spawning.

❌ Prolonged "preventive" treatment → stressed discus. 5-7 days maximum, then a break.

Prevention — how to avoid Saprolegnia

See also Weekly aquarium maintenance and Decor / hardscape.

Common primary causes of Saprolegnia in discus

When to consult a specialist

• Recurrences after 2 complete courses
• The whole group affected at once (rule out a primary disease)
• Saprolegnia + a secondary bacterial infection (red edges appear)
• Affected fry (sensitive dosing)
• Diagnostic confusion with Columnaris (when in doubt)

Post on dedicated forums with clear photos (in natural light) + water parameters + recent history + the protocol you followed — or contact breeders from the Discus.ro directory.

Final checklist — managing Saprolegnia

• ☐ Diagnosis confirmed (fluffy 3D white cotton wool — NOT flat yellowish patches)
• ☐ Differentiated from Columnaris (see table)
• ☐ Primary cause identified (wound? disease? water?)
• ☐ Hospital tank set up (40-80L / 11-21 US gal with a mature sponge filter)
• ☐ Activated carbon removed
• ☐ Started Methylene Blue 2 mg/L / JBL Fungol / salt 3 g/L
• ☐ Stable temperature 29°C (84°F)
• ☐ Maximum aeration
• ☐ 25-50% change / day with re-dosing
• ☐ Treatment 5-7 days
• ☐ Checked recovery daily
• ☐ Removed the primary cause (decor / aggressor / water)
• ☐ Post-treatment: new carbon + a vitamin supplement for 2 weeks
• ☐ Catappa leaves for ongoing prevention

What to do next

📚 Continue learning:

• Columnaris in discus — the critical differentiation
• Fin Rot in discus — common primary wounds
• The essential aquarist's pharmacy — the complete kit
• Weekly maintenance — prevention #1

🔧 Practical resources:

• Treatment dosage calculator — Methylene Blue, JBL Fungol
• Visual diagnosis — at the first signs
• Aquarium treatments — shop

🐠 Final tip: Saprolegnia is rarely the disease, more often the symptom. Before the antifungal, ask yourself WHY it appeared — a wound? water? disease? Treat the cause, not just the cotton wool. With discus breeders, Methylene Blue in the breeding tank is a global standard — every Stendker and Asian breeder uses it at each spawning. For a healthy aquarium with regular changes and decor without rough edges, Saprolegnia is rare — it appears only when the environment declines.