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Thread: Maintaining FC level and other questions

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    Maintaining FC level and other questions

    Thanks to the great advice I've gotten here (as opposed to the pool store) I feel like I finally have a reliable source of advice about managing our pool. So of course I'm back for more advice.

    We're fighting black algae in our white plaster pool. Have been shocking the pool with liquid chl for more than a week. My goal is to keep the FC level at 36 ppm.

    We are brushing w/SS brush and wall whale as much as possible- at least once a day weekdays, twice a day on weekends. This is the most we can brush since I'm not physically able to do the brushing and my spouse leaves for work about 5:30 am and can only brush the pool after work about 6 pm.

    We have been running the main pump 24 hrs a day and running the pool sweep 3-4 hours a day during the day to help keep the water stirred up.


    1) The last 2 nights we have only lost 1-2 ppm overnight. Night before that we had a drop of 9 ppm. Does this mean we are probably making progress with actually killing the algae?

    2) I'm assuming it's too soon to see any change in the existing bloom (all little dots - largest about 1/8" diameter but present in many places in the pool). Anyone have any idea how long we will probably need to keep this up before we see improvement?

    3) During the day the last two days we have only lost 4-5 ppm between the morning test and the evening test I do in order to determine how much chlorine to add to the pool before nightfall. So the daytime FC is only falling to 31-32 ppm. Do you recommend that I check the pool midday and adjust the FC up to 36 again?

    4) Can I safely reduce the amount of time I run the pump now that the FC levels are stable overnight? If so how many hours a day would you suggest and how would you split it up (day vs night)?

    5) Last question. Can I rely on my pH and TA test results using a DPD test kit when the FC levels are this high?

    Thanks so much,


    15,000 gal, IG, white plaster, cart filter, jet vac sweep, new pumps and filter system this year, water sparkling clear

    FC 36
    CC 0
    ph 7.5
    TA 110
    CH 280
    CYA 50
    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

  2. Back To Top    #2
    JasonLion's Avatar
    Join Date
    May 2007
    Silver Spring, MD

    Re: Maintaining FC level and other questions

    1) Progress, yes. Finished, no.

    2) It can be a little tricky to tell when you are done, since the black dots don't always go away completely. At the very minimum you need to go till the FC level stays the same overnight, but adding a couple of extra days at shock level to that is usually a good idea.

    3) At this point adding chlorine once a day should be plenty.

    4) I would leave the pump running. Any reduction in circulation could allow the black algae to lower the local FC level and start growing again.

    5) The PH test is useless at very high FC levels. The TA test tends to go blue/yellow instead of green/red but otherwise works even at very high FC levels.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
    Creator of PoolMath and Pool Calculator. Other handy links: Support this site, TF Test Kits, Pool School

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