What It Is:

Potassium permanganate is a purple granular powder. It can stain your hands brown if contacted directly.

What It's Good At:

Potassium Permanganate (KMnO4) kills 99.9% of exposed pathogenic bacteria, kills most ciliate parasites, most fluke parasitisms, fungal infections and potassium permanganate oxidizes organics in the water, leaving it cleaner and fresher than before treatment.

Here are some guidelines:

For flukes you should treat every third or fourth day for three or four treatments.

For ciliate parasites you should treat every day for five treatments.

Background:

Powerful oxidizer clears many ciliated protozoans, flukes, fungi and bacterial infections. Dosage and interval are critical. Use caution and dose carefully, avoid contact with skin, eyes or mucous membranes. Bypass filter and increase aeration and circulation. Can be neutralized with dechlorinator or hydrogen peroxide. Full details with product.

Pros:
There's hardly anything with such a broad spectrum and this can be used in extremely cold water.

Cons:
If carelessly overdosed, it will kill your fish. In addition to waxing most exposed pathogenic bacteria, it also kills 99.9% of exposed beneficial bacteria so you're advised to avoid letting it pass through your filter.