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Thread: Low chlorine levels

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    Join Date
    Nov 2015
    Los Angeles, CA

    Low chlorine levels

    Hi all, within a day or two of adding a gallon of liquid chlorine to my pool my reading is back at zero. Took a sample to my local pool shop and they said everything looks good except for the low chlorine level. They said a gallon a week should be sufficient for my in-ground 19,000 gallon pool. Because the pool was painted, my pool guy suggested not using tablets and only using liquid chlorine. What could be causing the low reading? Should I maybe add half a gallon twice a week instead of the full gallon at one time? The water is clear and the pool appears clean.

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    Mod Squad JohnT's Avatar
    Join Date
    Apr 2007
    SW Indiana

    Re: Low chlorine levels

    Chlorine is added on an as needed basis. You should be testing and adding chlorine at least daily. Bather load, use of covers, CYA level, climate and pool size all determine how much chlorine your pool will use. You will typically lose 2 to 3ppm per day, but it varies.
    TFP Moderator
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    Join Date
    Jan 2014
    Buckeye AZ

    Re: Low chlorine levels

    The pool guy is right in that tabs can concentrate chlorine in one area. They also add stabilizer with every tab. Over time your stabilizer will be so high that you can't keep the FC level high enough to compensate.

    A gallon could very well last you a week, especially in the winter. However, if you add the whole gallon at once, then most of that chlorine will be unprotected by your stabilizer, and will burn off much faster. TFP method is dosing every day, to keep your FC at a level sufficient to kill algae and sanitize the pool, but not add more than you need. Your stabilizer (CYA) level determines how much of your FC is protected. If you are over that line you are wasting chlorine, if you are under that line you are at risk for algae.

    See the pool school links at the top for info about the CYA/FC ratio, and get yourself one of the recommended kits. You will never need the pool store or pool guy again, and will spend only a few hundred a year in supplies.
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    Mod Squad pooldv's Avatar
    Join Date
    Aug 2012
    DFW, TX

    Re: Low chlorine levels

    Definitely get a good test kit, I use the TF100 from Or a Taylor K-2006.

    Here is a great place to start learning about taking care of your pool.
    Pool School - ABCs of Pool Water Chemistry
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    Richard320's Avatar
    Join Date
    Jan 2010
    San Dimas, CA (LA County)

    Re: Low chlorine levels

    If your CYA level is low or nonexistent, your gallon of chlorine will be gone in a few hours.

    This time of year, with reduced sunlight and cooler temperatures, that gallon should be close to if not more than enough for a week.

    Two things consume chlorine: sunlight and organics. Sunlight would be the low CYA. Organics could be a bunch of debris in the pool (or in the filter or pump strainer since they all share the same water even if you don't see it) or algae. The surefire way to tell is with an overnight loss test. To do that, you need a proper test kit. Odds are about 50:1 against you finding what you need in stock at a local pool store. You want a FAS-DPD test kit. Do not let a pool store tell you the DPD test is the same. It's not. A proper test kit will also let you read the CYA level.

    Pool School - Test Kits Compared
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