Household bleach is already diluted to a 5% solution - check the container. EPA recommends adding bleach to drinking water that is going to be stored for any length of time. Treating the fresh water tank is the easiest way to insure that the water that is left in the hot water tank after draining does not have a problem later with bacteria as the drain sits high and the last two gallons stay inside unless you use an external pump.
4 teaspoons of household bleach is enough to protect 20 gallons of drinking water for months. I do this for all water we add while traveling as I know better than to blindly trust what comes out of a public tap much less a plastic bottle.
