When fish have visible external parasites such as lice and leeches, or show signs of parasitic infection (such as flicking, rubbing or gasping) or ulcers.
Permanganate can be used as a whole pond treatment to treat parasitic infections from Costia, Trichodina, Chilodonella, etc, or as a dip treatment in a separate volume of water to treat fish lice and leeches, and for cleaning ulcers.