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Thread: quick question about algae in a SWG/ sand filter pool

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    Join Date
    Jul 2013

    quick question about algae in a SWG/ sand filter pool


    I was looking over the pool calculator, which i've been using this year to manage the levels in my pool. I'm a little confused having come across this chart on the relationship between free chlorine and CYA levels..

    I plug in 60 cya into the pool calculator and at the bottom it lists my shock level at 18. But on the chart above it says for 60cya the shock level should be 24.

    I'm quite frustrated at the algae mocking me.. I just turned on the new swg and sand filter.. it was crystal clear for about 2 weeks and then I woke up to an algae bloom.

    Another thing is that my manual tells me to have 30 cya in the pool.. so i'm confused at the recommended level of 60 on the chart here. I've been keeping around 30 in the pool the entire time and my free chlorine level has been floating around 7, which is above the recommended level.. so i'm confused at the algae appearance.. i thought if we had adequate free chlorine in relation to the CYA we'd be fine..

    my other numbers are about
    Alk- 90
    PH- 7.6 (though it likes to rise to 7.8 on me so I add acid to drop it almost every other day)
    CYA- 30
    FC- 7- 7.5 (though right now I shocked it with bleach and have it at around 13 with the swg pumping even more in)

    I think i've come to the realization i'm going to have to shock the pool often even with this SWG/Ozone generator.. we live under neighbor's tree so i have leaves and anything on the tree that falls into the pool to deal with.. I imagine the process is much easier if your pool is out in the open.. I just have to find my method given my pool location... I don't mind shocking, I just don't like being caught off guard with it like it did with me this time.. I was feeling all proud with the new sand filter and swg.. given we're coming from an intex cartridge filter that did nothing last year.. but the algae keeps mocking me..

    I just don't get how my FC level of 7.5 with the swg pumping out about FC10 or so while running could lead to an algae bloom.. i've been keeping the pump off for about 6 hours overnight.. I didn't think i had to run it overnight.. I did order phosphate test kit to see for the sake of it what that level is.. I need all the help I can get in preventing algae.. it's a pain doing the slam process with family wanting to use the pool.. I need to more prevent it than treat it..
    Intex 18' x 48" above ground
    Intex 2650gph sand filter
    Intex SWG/ Ozone generator

  2. Back To Top    #2
    bobodaclown's Avatar
    Join Date
    Apr 2011
    Murrieta, CA

    Re: quick question about algae in a SWG/ sand filter pool

    Having the CYA at 30 the sun is burning off the chlorine quicker than it can produce and keep the pool sanitized (residual FC level). You can follow the advice here or what's provided by the book, but follow both you will have troubles.
    Murrieta, CA. 15K (Est) pool/spa. Hayward Tristar SP3220EE 2.70HP pump, Hayward Swimclear Cartridge filter C4025 425sqft (PA106), Pentair Mastertemp 400 heater. TF-100/Speedstir

  3. Back To Top    #3
    Smykowski's Avatar
    Join Date
    Sep 2011
    Gurnee, IL (North Suburban Chi-town)

    Re: quick question about algae in a SWG/ sand filter pool

    Any discrepancy in the FC/CYA ratio has been fixed in PoolMath (link at the top of the page). The old pool calculator was abandoned by its owner and we had no control over it.
    33' round, 23,000 gal AG vinyl , 1HP 2spd PowerFlo Matrix downsized with 3/4HP impeller (X2), Hayward S180T 150# sand filter (X2), Hayward H250 NG heater Pool Store year 1 - $850 for 2 months; Pool Store year 2 - $440 for 2 months, TFPC year 1 - $170 for 4 months; TFPC year 2 - $95 for 4.5 months
    The most important article on this site - The ABCs of Pool Water Chemistry

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