I would be careful when using potassium permaganate. Its extremely reactive (P.P. + Glycerine = fire....just an example of its reactivity)stuff and if dosed wrongly could kill your fish. Having that said, you should only use P.P. if you are excersicing real caution.
First off, you should never put P.P. straight into your tank. You should prepare a bath for whatever needs to be treated, and dip in the things that need to be dipped. As far as dosage goes, you should put as much in (with a little toothpick or cotton swab) untill the water turns a color between pink and purple. It is ideal to weigh out the stuff before adding since guaging by color is usually not as accurate as we wish. 0.05 mg/L or potassium permaganate is the safe dosage, but since most people dont have sensitive enough scales that can read .0005 grams of any substance, we have no choice but to use color as an indicator. You should keep the specimen in the solution for about 10 minutes. If your dipping fish that have open wounds, dont leave them in there for too long. If you do get P.P in the tank, i think a good way to neutralize it (and also get rid of the color) is by using 3% solution of hydrogen peroxide. Honestly ive never tried this but ive heard an alternative of using dechlorinator instead of hydrogen peroxide.
Another, safer, way to kill unwanted pests is by preparing a hydrogen peroxide (H2O2)dip. This is done by adding 1 mL of 3% H2O2 per 10 mL of water. Dip for the same amount of time you did with the potassium permaganate solution.
Again make sure to excercise extreme caution because any miscalculations in the dosage of P.P. or H2O2 can cause death to any of the things you are dipping. I hope this helped
