I think the ratio is someting like one tablespoon bleach per gallon of water. Therefore less than 1 gallon of bleach should treat 1000 gallons. However, I would find out what he really should put in the water? I would prefer using clorine like what you put into a swimming pool. Also, get a clorine tester & you will be able to test the amount of clorine in the cistern. If he's not using the cistern water for drinking and cooking then there is less of a concern. Next question. How often to add bleach or clorine?