usually 2 cristals per liter to kill nasty stuff.

personally i do not use kmno4 for anything because:
-there arent many reseach about kmno4 and purfication, so we dont know about the dangers and stuff like that.
-lethal in large quatity ( swallowed )
-can cause burns on the skin wenn it is repeatedly used as an antiseptic ( or exidental spill on the skin of an large quantity )
-can cause instand blindness wenn in contact with your eye's
-hard to dose.
-hard to optain ( atleast for me )