Columnaris: what it is and what to do

Columnaris is a bacterial infection caused by Flavobacterium columnare. It’s often called cotton wool disease or saddleback disease because of how it looks — white or grey fuzzy patches, sometimes in a band across the fish’s back. Despite the nickname “mouth fungus,” it’s not a fungus: fungal treatments won’t work. You need an antibacterial approach.

The infection can spread quickly and kill within days if left untreated. Early signs include white or grey patches on skin, fins, or mouth; frayed fins; and fish gasping or breathing fast if the gills are affected. Stress, poor water quality, overcrowding, or new fish introduced without quarantine often trigger outbreaks.

Our full guide to columnaris covers symptoms in detail, how to treat with antibiotics (gram-negative), and how to prevent it. See also fin rot, ich, top fish illnesses, hospital tank, and quarantine.