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Thread: Chlorine tablets causing brown stains ??

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    Join Date
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    Question Chlorine tablets causing brown stains ??

    I needed to leave my pool unattended for a month. It's leaf season, so I decided to disable the pump and leave it dormant as there would be nobody around to empty the skimmer etc. every day. As an "extra precaution", I dropped in 10 or so chlorine tablets at various locations around the pool. I got back to find all OK, except that I now have a big nasty brown stain around the remains of each chlorine tablet. Like the chlorine tablets were big rusty bolts or something. I'm suprised by that ?!? I've heard of using chlorine tablets to remove stains, but not to cause them. Any idea what's going on ?

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    Mod Squad tim5055's Avatar
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    Re: Chlorine tablets causing brown stains ??

    What kind of pool? Liner or cement?
    TFP Moderator 39 X 18 23,000(ish) freeform gunite; built 2007ish; Pentair Triton II TR100 600lb Sand filter; 2 HP Pentair pump with 2.2 HP AO Smith single speed motor; 2 skimmers, 1 main drain, 4 returns w/waterfall, Stenner 45MHP2 3GPD running@ 60% - 15 gal Tank; heated by the sun CYA 200+ when I started - 50 now. Dolphin Supreme M5 Pool Cleaner. Hot Springs SX Spa, 285 gallon

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    Re: Chlorine tablets causing brown stains ??

    In-ground gunnite pool with white plaster finish ...

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    Mod Squad tim5055's Avatar
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    Re: Chlorine tablets causing brown stains ??

    Chlorine tabs are very acidic and it is not recommended that they just be tossed in the pool because they can damage the pool surface. If they are still in there I would recommend using your net and getting them off the bottom and into a floater.

    It's possible that the surface under the tab locations may just be very clean, surrounded by existing stains or there may be damage to the gunite. Unless you can get down there with a mask and take a look it would be hard to tell.

    How much have you read in Pool School ?

    I would start with - Pool School - ABCs of Pool Water Chemistry
    TFP Moderator 39 X 18 23,000(ish) freeform gunite; built 2007ish; Pentair Triton II TR100 600lb Sand filter; 2 HP Pentair pump with 2.2 HP AO Smith single speed motor; 2 skimmers, 1 main drain, 4 returns w/waterfall, Stenner 45MHP2 3GPD running@ 60% - 15 gal Tank; heated by the sun CYA 200+ when I started - 50 now. Dolphin Supreme M5 Pool Cleaner. Hot Springs SX Spa, 285 gallon

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    Re: Chlorine tablets causing brown stains ??

    Chlorine tabs do leave a brown stain which is iron, but are more difficult to remove than ordinary iron stains. Plaster contains a small amount of iron and that high concentration of chlorine and acid will turn the plaster surface a yellowish brown color.
    If the tabs are trichlor tabs, and left on the surface for a long time, then they will also etch the plaster surface.
    I think the only way to remove the brown stain and etched area will be to sand (with sandpaper) those areas and make the plaster surface smooth again.

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    Re: Chlorine tablets causing brown stains ??

    Thanks for the info. I'd never have guessed you could get iron stains from a chlorine tablet and plaster, but that's exactly what they look like. Y'live and learn.

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