Symptoms: Fungus is characterized by the growth of thin threads on the skin or the fins of fish. If fungus growth is  very abundant, it may resemble tufts of cotton.  At times the infection may deform the affected area.  Fungus may infect the fins, tail, body, or eye of the puffer.  *(take special note of mouth fungus, as it may actually be the bacterial infection, columnaris)

Treatment:Maroxy,Malachite green, formalin, Methylene blue(this may stain the silicone seal in your tank), salt baths, oxidation solutions.

Background:  Fungus is due to a great many kinds of molds of the family Saprolegniaceae. The most common are the species from the genera Saprolegnia and Achlya. They can only attack puffers which have been wounded or whose resistance has been weakend by other parasites or certain unfavorable conditions, such as (extreme) Temperatures or pH.  After infection of the puffer's skin, at first a mycelium will be formed, growing between the cells of the epidermis and penetrating from there into the cutis and even to the underlying muscles. Meanwhile hypea, (threads of which the molds consists), are formed which grow not only upon the skin but also through the skin and into the muscles. By their action the cells of the skin and muscles begin to degenerate and eventually die. In very severe cases the fungus might even penetrate into the skeleton. Puffs that are completely happy and healthy and undamaged do not get fungus, even though the spore might be present in the water. Were this not so, propably no fish would exist at all, since fungus spores are always present in nearly all natural waters where fishes live. Fungus is not an onligatory parasite. It can live very well without the presence of fish, provided that there are organic mater for it to live of.  Since fungus is not infectious to healthy fish, it isn't necessary to remove the infected fish from the community, although it is advised to give a suitable treatment. (Axelrod, Shultz 1990)