https://www.chewy.com/education/fish/general/aquarium-fish-mouth-fungus
Despite its name, mouth fungus - also known as columnaris - is not a fungal infection but a bacterial infection, requiring treatment with antibacterial or antibiotic medications. While it often appears around the mouth, it can also affect the gills, face, jaws, and occasionally the body and fins, which can lead to confusion with fin rot.
Mouth fungus is typically identified by its short, off white to gray tufts. This sets it apart from true fungal infections, which look more like long, fluffy white threads resembling cotton wool.
To complicate things, some hobbyists still refer to mouth fungus as “cotton wool disease,” even though true fungal infections are actually more cottony in appearance.
In advanced stages, mouth fungus can cause painful sores and lesions on the head and sides of the fish. It is rarely seen in well maintained tanks and is more common in poorly managed or overcrowded aquariums.
Prevention starts with proper tank setup and maintenance. This includes cycling the aquarium before adding fish, choosing compatible species with similar water and social needs, and avoiding conditions that lead to stress or injury - such as aggressive tankmates. Goldfish, for example, are especially vulnerable when kept in unfiltered or overstocked environments.
Chewy editorial team provides expert insight on identifying, treating, and most importantly, preventing mouth fungus to help ensure a healthy environment for your fish.
