The familiar and seemingly ubiquitous infections we've named Cotton Mouth, Fin Rot, Body Slime, Saddle back, Cotton Wool, Columnaris & Fungus, are usually the result of one or more, of two types of nasties, either fungal or bacterial in nature.
Both are common in the aquarium, it being a good petri dish, and given the chance they thrive on our fish.
Stress of all kinds, e.g. over crowding, fighting, drastic changes in environment and etc, and injury/illness, leads to suppressed immune systems, thus giving them the chance to take hold.
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These bacterial infections are the result of gram negative, aerobic, Flavobacterium, Flexibacter and/or Myxobacteria.
Bacterial usually appear as white/gray powder coating, slime and cob webbing, combined with skin/tissue lesions in live tissue.
Effective treatment(s) includes, fix whatever is stressing the fish, good water quality (NO3 below 20ppm), lowering temp to less than 75'F for duration of treatment, Pimafix (mild cases), Salt, Acriflavin (mild cases), Maracyn-2, Kanamycin & Furan-2 and other gram negative specific antibiotics.
Do not use gram positive medications such as Tetracycline, Erythromycin & Melafix.