How to Recognize and Treat Columnaris (Cotton Wool Disease)

How to Recognize and Treat Columnaris (Cotton Wool Disease) | Aqualista

🦠 Columnaris is a fast‑killing bacterial infection often mistaken for a fungus — This guide covers symptoms (cottony patches, saddleback lesion), treatment with antibiotics, and prevention through water quality.

What Is Columnaris (Cotton Wool Disease)?

Columnaris is a bacterial infection caused by Flavobacterium columnare. It appears as white or grayish patches on the skin, fins, or gills, often with a cottony texture. It can kill a betta within 24‑48 hours.

Despite its cottony appearance, columnaris is not a fungus – it is a gram‑negative rod bacterium. The Merck Veterinary Manual describes columnaris as one of the most common bacterial diseases in aquarium fish. It thrives in warm water (above 80°F) and spreads rapidly. The bacterium attacks the skin, gills, and mouth. There are several strains; some cause slow‑growing lesions, while aggressive strains kill within hours. Stress (poor water quality, temperature fluctuations, overcrowding) triggers outbreaks.

How to Recognize Columnaris in Bettas: Symptoms and Stages

Early: white or gray patch on the back, often near the dorsal fin (saddleback). Late: cottony tufts on the mouth (mouth rot), fins, or gills. Gill infections cause rapid breathing and lethargy.

The classic sign is a “saddleback” lesion – a whitish area across the back behind the head. The Merck Veterinary Manual notes that columnaris lesions often start as pale patches, then develop into raised, cotton‑like growths. On the mouth, it causes “cotton mouth” (similar to fungal infection). Gill columnaris causes labored breathing, rapid opercular movement, and fish gasping at the surface. In advanced stages, the skin may erode, exposing muscle tissue. Without treatment, death occurs in 1‑7 days depending on the strain.

🔍 Distinguishing columnaris from fungus – Columnaris: grayish/white, often flat initially, spreads rapidly, lesions may have a yellow edge.
– True fungus (Saprolegnia): fluffy, white/gray, grows in cottony tufts, slower progression.

What Causes Columnaris Outbreaks in Betta Tanks?

Poor water quality (ammonia, nitrite), sudden temperature changes, overcrowding, shipping stress, and wounds from netting or fighting. The bacteria is often present in tanks but strikes when fish are stressed.

Flavobacterium columnare is opportunistic. The Merck Veterinary Manual states that high organic loads (excess food, waste) promote bacterial growth. Stress suppresses the immune system. Common triggers: moving bettas to a new tank, temperature drops, high ammonia during cycling, or fin injuries from tankmates. Warm water (80‑84°F) accelerates the disease. In community tanks, columnaris can spread rapidly to other fish. Always quarantine new arrivals for 2‑4 weeks to prevent introducing columnaris.

🧪 Risk factors: pH below 6.5, hardness below 50 ppm, temperature above 82°F, and high ammonia or nitrite. Correct water quality before treating.

How to Treat Columnaris in Bettas (Antibiotics)

Use effective antibiotics: kanamycin (Seachem Kanaplex), nitrofurazone (Furan‑2), or a combination. Treat in a hospital tank for 5‑7 days. Add aquarium salt (1 tsp/5g) to reduce stress and aid healing.

Columnaris is resistant to many antibiotics. The Merck Veterinary Manual recommends kanamycin or a combination of nitrofurazone and kanamycin. Use a hospital tank (5‑10g) with a sponge filter. Remove carbon. Dose according to instructions. For Seachem Kanaplex, dose every 48 hours for 3 doses. For API Furan‑2, dose every 24 hours for 4 days. You can combine half doses of both for broad coverage. Add aquarium salt (1 teaspoon per 5 gallons) – salt reduces osmotic stress and inhibits columnaris. Raise temperature to 78‑80°F (not higher – columnaris spreads faster at high temps). Treat for 5‑7 days after symptoms disappear.

💊 First‑line treatmentSeachem Kanaplex (kanamycin) – effective against gram‑negative bacteria.

💊 Combination therapyFuran‑2 (nitrofurazone) + Kanaplex – for severe or resistant cases.

🧂 SupportiveAquarium salt (1 tsp/5g) – helps gill function and reduces stress.

Columnaris Treatment Protocol (Step by Step)

Day 1: Move betta to hospital tank, add salt (1 tsp/5g), start antibiotics. Day 2‑5: Continue antibiotics, 30% water change before each dose. Day 6: Observe. If lesions heal, discontinue meds with water changes.

Step 1: Set up a hospital tank with clean, dechlorinated water, heater (78°F), and a sponge filter (if possible, seeded). Step 2: Add aquarium salt – 1 teaspoon per 5 gallons. Stir to dissolve. Step 3: Add first dose of antibiotic (e.g., Kanaplex). Step 4: After 48 hours, do a 30% water change, then add second dose. Step 5: Repeat for a total of 3 doses (6 days). Step 6: Observe the fish. If the cottony patches are gone and the betta is eating, stop medication. Perform daily 30% water changes for 3 days to remove residual antibiotics. Step 7: Return the betta to the main tank only after water quality in the main tank is perfect (0 ammonia, 0 nitrite). Aquarium Co‑Op has a video protocol.

🩺 Severe columnaris protocol: Add a second antibiotic (Furan‑2) alongside Kanaplex. Use half dose of each to avoid overdose. Treat for 5 days, then reassess.

Can Salt Baths or Other Remedies Cure Columnaris?

Salt alone will not cure columnaris – the bacteria is resistant to salt at safe concentrations. You must use antibiotics. Salt is supportive, not curative. Do not use tea tree oil (Melafix) – it is ineffective.

Many home remedies (peas, garlic, methylene blue) are ineffective against columnaris. The Merck Veterinary Manual confirms that antibiotics are necessary. Aquarium salt at 1‑2 teaspoons per gallon may slow the bacteria but will not eliminate it. Epsom salt is for constipation, not for columnaris. Tea tree oil (Melafix, BettaFix) has no antibacterial activity against Flavobacterium and can damage the labyrinth organ. Do not waste time on ineffective treatments – start antibiotics immediately. If you cannot obtain antibiotics quickly, perform a 100% water change and add salt, but obtain antibiotics within 24 hours.

❌ Ineffective treatments: Melafix, BettaFix, Pimafix, aquarium salt alone, Indian almond leaves alone, raising temperature (makes it worse).

How to Prevent Columnaris in Betta Tanks

Maintain pristine water quality (0 ammonia, 0 nitrite). Quarantine new fish for 2‑4 weeks. Avoid sudden temperature changes. Reduce stress with Indian almond leaves and hiding spots. Do not overcrowd.

Prevention is far easier than treatment. The Merck Veterinary Manual emphasizes that columnaris outbreaks are almost always triggered by poor conditions. Test water weekly; keep nitrate below 20 ppm. Perform 25‑30% water changes weekly. Quarantine all new fish – columnaris can be carried without symptoms. Use a UV sterilizer in the main tank to reduce bacterial load. Avoid buying fish from tanks with visible cottony lesions. Keep the betta’s immune system strong with varied diet (pellets + frozen foods). A single Indian almond leaf per 5 gallons provides antibacterial tannins that may help prevent columnaris.

🛡️ Prevention checklist – Weekly 25% water changes
– Test ammonia and nitrite weekly
– Quarantine new fish (2 weeks minimum)
– Add Indian almond leaves
– Feed high‑quality varied diet
– Avoid temperature fluctuations

When to Euthanize a Betta with Advanced Columnaris

If the betta has large open ulcers, refuses to eat for 5+ days, or has severe gill damage (gasping at surface with red gills), euthanasia with clove oil may be the kindest option.

Columnaris can cause extensive tissue necrosis. Once the bacteria reaches the internal organs or the gills are severely damaged, recovery is unlikely. The Merck Veterinary Manual notes that advanced columnaris has a poor prognosis. Signs that treatment has failed: lesions continue to grow despite antibiotics, the fish stops eating, lies on the bottom, or breathes rapidly with open mouth. If there is no improvement after 3 days of antibiotics, consider switching to a different antibiotic. If the betta is visibly suffering and unable to swim, euthanize using clove oil: 0.4 mL per liter of water. After euthanasia, disinfect the hospital tank with a 10% bleach solution. Do not reuse equipment without sterilization.

📋 Recommended products for columnaris (nofollow):
• Seachem Kanaplex (kanamycin)
• API Furan‑2 (nitrofurazone)
• Aquarium salt
• Aquarium Co‑Op – broad spectrum antibiotics

📚 Explore more betta health and treatment guides: ← Main Betta Hub (T1) Disease Hub (T2) Hospital tank Aquarium salt guide Popeye treatment You are here Fin rot guide

📊 Columnaris vs Fungus – quick comparison

Feature	Columnaris (bacterial)	Saprolegnia (true fungus)
Appearance	Gray/white patch, flat or slightly raised, may have yellow edge	Fluffy, cottony, white/gray, fuzzy
Location	Back (saddleback), mouth, gills, fins	Often on existing wounds, dead skin, or eggs
ProgressionRapid (24‑48 hours can be fatal)	Slower (days to weeks)
Treatment	Antibiotics (kanamycin, nitrofurazone)Antifungals (methylene blue, salt)
TemperatureWorsens with heat (>80°F)Slows with heat (above 80°F)

📖 References & further reading
• Merck Veterinary Manual – Columnaris
• Aquarium Co‑Op – Columnaris treatment guide
• Bettafish.com – Columnaris case studies
• Seachem Kanaplex – antibiotic information