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Thread: Increase in CYA following rain???

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    Increase in CYA following rain???

    I have been successfully maintaining the CYA level in my pool between 40 and 50 for several months. We keep our FC levels a little high because of a hx of black algae. This has worked well for several months.

    So I had stopped testing the CYA level frequently since it had held steady and I have NO sources of CYA adding to the load. I use liquid chlorine only.

    Recently we had a series of thunderstorms that added maybe 1" or 2" at most to the pool. I didn't think to check the CYA level afterwards because a slightly lower CYA level would be fine. It's higher I don't want to do. Did have to adjust the pH down a little more than usual.

    So about 5 days after the rain we notice a resurgence of the black algae. Very small spots but definitely an increase in growth.

    I raised the chlorine level up to 30 for 3 days (FC stayed steady overnight) and did heavy brushing, etc. and the algae retreated but then resurged again once I dropped the FC back down to 8.

    So I just checked my CYA level (had to go buy more reagent) and it's up to 90!!!

    The ONLY thing that could have done this was the rain. But I thought rain diluted CYA not increased it.

    Anyway, I am doing a partial drain and refill but was wondering if anyone else had seen this.

    After refill I will recheck and adjust numbers as needed. Probably drop the pH to 7.2 and increase FC again.


    Plaster pool 17000 gal, Hayward pumps w/4 cart filter, Jet-vac sweep

    TC 8
    FC 8
    pH 7.5
    CYA 90
    CH 250
    TA 80
    17,000 gal IGP, White plaster, Hayward Swimclear Cartridge Filter system, Sta-Rite Max-e-pro 1/2 hp pump; 3/4 hp booster pump

    Sacramento, CA

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    frustratedpoolmom's Avatar
    Join Date
    May 2007
    SWSuburban Chicago, IL

    Re: Increase in CYA following rain???

    Sounds like someone added CYA somehow, via a granular shock product or tablets.

    Rainwater would dilute the level, not increase it. Testing error is always possible - did you do the test under the same lighting conditions everytime? The test is so subjective....
    Helpful links: Pool School; CYA/Chlorine Chart
    24' round AG pool, 52" high, Raypak heater; Waterway 2 spd Pump;
    150 Sq ft. Clearwater Cartridge Filter; Former and DISSATISFIED "Pool Frog" owner
    Perfectly Clear Pool Service, Find me on Facebook!

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    duraleigh's Avatar
    Join Date
    Apr 2007
    Sebring, Florida

    Re: Increase in CYA following rain???

    Sounds like someone added CYA somehow, via a granular shock product or tablets.
    Yep. CYA does not occur in nature. The only way to get CYA in a pool is for someone to somehow put it there.
    Dave S.
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
    TFTestkits , PoolMath , Pool School

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