Symptoms: Cottonmouth appears as a tuft of cottonlike material on the mouth or cheek region of the pufferfish.  The puffer's lips may become swollen, and its appetite decreases or stops altogether as it becomes sluggish.

Treatment: Remove new carbon from your filter.  Administer antibiotics.  Maracyn-two is recommended, although many antibiotic agents such as penicillin and tetracycline have some success.  Follow the directions on the package.  Aquarium salt is also helpful. (roughly 1 tablespoon per 5 gallons for freshwater puffers, temporarily)  Since this condition is contagious, quarantine the tank and be wary of contamination via nets, hands, etc.

Background:  Cottonmouth is misleadingly referred to as "mouth fungus".  While similar in appearance, it is actually the Chondrococcus columnaris (or Flexibacter columnaris) bacterium which is responsible.  The bacteria take hold in the mouth and/or cheek tissue of the puffer, spreading to the other parts of the head region if not properly treated.