I would definitely use something biological before trying potassium permanganate. 1) it's hard to get ahold of in a pure form, 2) it's a very stong oxidizer (to the point of starting fires) and will burn your skin on contact, I'd imagine it'd do the same to the coral and that's probably what kills the nudis. 3) It is occasionally used to treat infections in humans using a concentration of up 0.05%. Attaining this low a concentration without volumetric glassware and an analytical balance is impracticable. 4) It's deep purple in color and will stain anything. Stick with fish or manual removal. Much safer all around. The fish might eat your coral, but at least they won't burn your fingers or your house down.