Household bleach from the local supermarket has already been diluted to a 5% solution. 2 cups in 30 gallons of water takes the 5% solution to a 0.02 percent solution.
Per the EPA for drinking water that is going to sit for months all you need to add to 30 gallons of water is 6 ounces of household bleach. The only time to use more is if you get a bacterial buildup as in the hot water tank and get the sulfur smell in which case you can shock the tank by using 4x the bleach in the FW tank and running the water through the hot water tank and letting it sit overnight and then drain everything (though you will not be able to completely drain the hot water tank unless you use a pump to remove all the water).
