Half a gallon? That's not nearly enough... you should be drinking half your weight in ounces of water.. For example, say you weigh 200lbs, you should be drinking 100 ounces of water a day...
Please be careful, that amt I said is just regular water consuption, now I have done small Water Fasts and I drank atleast that or more. If you don't drink enough your body will never be able to flush the toxins from fasting...