I've tried a ton of crap, most of which didn't work completely as advertised or in fact gave me the craps. I found small quantities of common bleach works best in rough locals. Easy to get, easy to use and cheap as hell. I ran into one of the leading Navy bio-tech researchers and he concurred with my opinion.
Common bleach is 5% sodium hypochloride, check the bottle you use to verify this strength. I dilute a cap full of bleach to 5 gallons on average, depending on how bad the source water is screwed up. 1 cap full (approx. half ounce) to a gallon has been my max. I give the bleach at least 6 hours to do it's stuff, overnight is better.