Mouth fungus is an odious illness, and is, luckily, rather rare. When one fish has it, it can contaminate all of the fish in the exact same tank, as it is extremely contagious. The illness becomes apparent when a cottony fluff appears around the lip and mouth, which soon gets into the mouth and eats the jaws away.
Owing to the extremely contagious nature of the illness, instant action is known as for. Regrettably, very small is recognized about the illness, and no treatment has been devised to treat the fish en masse. Dab the lips with cotton wool soaked in peroxide of hydrogen (20 vols) straight from the bottle, or mentholate. Deal with the fish carefully, hold it in an absorbent soft linen thoroughly soaked in water, or in a wet net. Salt baths may also be tried, but the actual effect of the salt has not yet been proved. A minimum of it has no harmful effect.
Fish most likely to develop this kind of fungus are those which have suffered bruised lips whilst being transported.