Columnaris: what it is and what to do

What is columnaris?

Columnaris is a bacterial infection caused by Flavobacterium columnare. It’s often called cotton wool disease, saddleback disease, or mouth fungus — though it’s not a fungus. The bacteria live in many freshwater environments and can infect fish when they’re stressed or injured. It spreads quickly and can kill within days if left untreated, so early recognition matters.

Columnaris vs fungus

Columnaris is easy to mistake for fungus because both can show as white or grey fuzzy patches. True fungus (Saprolegnia) usually grows on dead tissue or wounds as a single, cotton-like tuft. Columnaris often appears as a broader band or saddle across the back, or as patches around the mouth, fins, and gills. Fungal treatments won’t work on columnaris — you need an antibacterial approach.

What does columnaris look like?

Symptoms vary by strain and how far the infection has spread:

• White or grey patches — Fuzzy, cotton-like growth on skin, fins, or mouth.
• Saddle-shaped lesion — A band across the back behind the dorsal fin (hence “saddleback”).
• Mouth lesions — Erosion or white film around the lips (often called mouth fungus).
• Frayed fins — Fin edges can look ragged or receding, similar to fin rot.
• Gill damage — In gill infections, fish gasp at the surface, breathe fast, or show pale or discoloured gills.
• Lethargy and loss of appetite — Fish may hide, stop eating, or sit on the bottom.

If you see these signs, check your water parameters and consider isolating the fish in a hospital tank.

What causes columnaris?

The bacteria are common in water and on fish. Outbreaks usually happen when something weakens the fish:

• Stress from poor water quality (ammonia, nitrite, or high nitrate).
• Temperature swings or water that’s too warm (columnaris thrives above about 75 °F).
• Overcrowding or aggression.
• New fish introduced without quarantine.
• Physical damage from nipping, rough decor, or handling.

How to treat columnaris

Act quickly. The infection can progress fast.

1. Isolate affected fish — Move sick fish to a hospital tank to protect others and make treatment easier.
2. Improve water quality — Test ammonia, nitrite, and nitrate. Keep ammonia and nitrite at zero. Do a partial water change (30–50%) with dechlorinated, temperature-matched water.
3. Use an antibacterial medication — Columnaris is gram-negative. Use a product labelled for gram-negative bacteria (e.g. Kanamycin, nitrofurazone, or similar). Remove carbon from filters during treatment. Follow the label exactly and complete the full course.
4. Salt (where safe) — Low-dose aquarium salt can help in early cases and support fish. Not all species or plants tolerate salt — check before using.
5. Lower temperature if possible — If your fish can handle it, reducing temp slightly can slow bacterial growth. Don’t shock the fish with sudden changes.

Log your parameters and treatment steps with a tool like App-aquatic so you can track progress and stay consistent.

Prevention

Reduce stress and keep water in good shape:

• Quarantine new fish — See our quarantine guide. A few weeks in a separate tank helps catch and treat disease before it reaches your main tank.
• Stable water quality — Regular testing and water changes. Avoid ammonia and nitrite spikes.
• Avoid overcrowding — Give fish space and compatible tank mates.
• Minimise handling and stress — Acclimate new fish properly; avoid sharp decor that can injure fins.

Quick takeaways

• Columnaris = bacterial infection (not fungus); looks like cotton wool or a saddle on the back.
• Treat with an antibacterial for gram-negative bacteria; isolate, improve water, complete full course.
• Prevent with quarantine, good water quality, and stress reduction.