To raise the chlorine level to shock level in my 20k gallon pool I figure it would need 8 jugs of bleach at 3 bucks each, that's $24.00. I was looking on the online pool stores and they sell 1lb bags at 2.75 per bag and claim 1 bag per 10,000 gallons. so 2 bags for my 20k gallon pool at 2.75 per bag that's 5.50. I'm new at this so please let me know if I am being misinformed. Please educate me. I completely understand the bbb method and it is working awesome. My water has never looked this good and it is so simple to do. I still have my chlorine feeder in place and thought it would be good to use when I go on my week vacation. I am currently having to pour in 1 jug a day . I'm thinking the 3" tabs would work while im gone. How long does a single 3" tab last in a feeder. I know it will raise my cyn level. This is a outdoor above ground pool. how long does it take for the the cynic acid to go away. I know at the beginning of the year my level is always zero.