Potassium permanganate is a purple powder that turns brown when it comes in contact with anything organic. It is an oxidizer, and classified as an explosive. It will stain your hands brown very quickly so wear gloves. I have heard it is used to clean vegetables so you might find it at an agriculture store. I found somebody that used it to regenerate water softener media, and got a container of it from them.
I used it to kill Montipora Eating Nudis over a year ago. Turned my montis brown for a couple of weeks, but killed the nudis and eggs.
Do you apply it directly to the fish's body? I would think a bath would damage the gills pretty quickly.