If water sits in the holding tank bacteria can grow in it over time. I carry a 8oz bottle of household bleach (which straight from the bottle is already 95% water). A tank in regular use only needs 1/2 ounce of bleach for every 100 gallons of water. If the water is going to sit for months then the amount of bleach to add is 1.6 ounces per 100 gallons of water or about three times the concentration.
This is from the FEMA guidelines for using and storing drinking water. The treated water is safer to drink than any bottled water you can buy in the store and the cost is pennies instead of dollars per gallon.