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Thread: Do I need to Shock to get rid of Black Algae - or is brushing enough?

  1. #1
    Join Date
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    Default Do I need to Shock to get rid of Black Algae - or is brushing enough?

    My pool has black Algae. It's a 20,000 gallon in-ground SWG pool in central Fl. I didn't notice that the SWG stopped producing chlorine because the salt level was too low. I noticed when I saw a lot of green algae in the pool. I fixed the problem and the green algae was easy to clean. However I'm having a hard time getting rid of the black spots.

    PH is 7.4
    ALK is 120
    CAL is 220
    CY is 90
    FC is 5
    TEMP is 90
    SALT is 4000

    Now that the Chlorine is back in the pool, do I just keep brushing the black spots with my SS brush for a couple of days/weeks - or do I need to shock the pool to get rid of the spots?
    When I brush the spots now (I've been brushing twice a day for 3 days now), I don't really see any immediate change (lightning or reducing in size) but I do think the overall problem is less bad now (less spots).
    -Mike
    20K in-ground pool, Pentair FNS60Plus Filter, IntelliChlor, IntelliTouch, IntellipH, SAM/SAL, two WhisperFlow 1.0hp pumps, Water Falls, RayPak, Jandy

  2. #2
    swimdaddy is offline *Removed User* Weir Watcher swimdaddy 0
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    Default Re: Do I need to Shock to get rid of Black Algae - or is brushing enough?

    With a CYA level of 90, the MINIMUM chlorine level is 5, maximum of 10, according to the Best Guess chart. The shock level is 20+ ppm. With algae in your pool, It is recommended that you get your FC to shock level until the pool is clear, the CC is less than 1 and you maintain FC overnight. Just brushing is not enough, though it is essential. Raise you FC using bleach, not your SWCG.

  3. #3
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    Default Re: Do I need to Shock to get rid of Black Algae - or is brushing enough?

    You said I need to increase to shock level until pool is clear" but Pool water is crystal clear - except for the small black spots on some places on the bottom and on some of the steps.

    Maybe you meant by "clear" - no black spots.

    So I should increase chlorine to 20+ ppm until black spots are gone?
    -Mike
    20K in-ground pool, Pentair FNS60Plus Filter, IntelliChlor, IntelliTouch, IntellipH, SAM/SAL, two WhisperFlow 1.0hp pumps, Water Falls, RayPak, Jandy

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    swimdaddy is offline *Removed User* Weir Watcher swimdaddy 0
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    Default Re: Do I need to Shock to get rid of Black Algae - or is brushing enough?

    If the black spots are algae, and you are having trouble getting rid of them, then you should bring your FC levels up to shock level, which for you is 20+, and keep it there while you brush and test daily until the algae is gone. You would then do an overnight test and if you lose no more than 1 ppm chlorine, you can let your chlorine levels drop to 5-10 ppm.

    Do you have a reading for CC? If its 1 or more you need to shock. Interestingly I have found that even with a CC of 0.5 or less, if the pool looks like it has algae, it probably does. Do you have an excessive chlorine demand? If you are putting more chorine in the pool than usual, something is using it up and you need to shock.

  5. #5
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    Default Re: Do I need to Shock to get rid of Black Algae - or is brushing enough?

    Thanks for your help.

    I didn't measure the CC, so I will do that and report back (I have Ben's test kit).

    Since the pool is a SWG, I don't pay much attention to how much Chlorine is going in or what the demand is. I just "adjust" the SWG so that I have chlorine in the pool (it's set at 30% now, and I run the filter/SWG 8 hours a day).

    My pool is not screened, so it gets direct FL sun, and the water temp this time of year is around 90 degrees, and I get a lot of afternoon rain. So it's a pretty tough environment for a pool. But oddly, even with all the rain, I can't seem to get the CYA under 90 (over the past couple of years).

    The problem started because I failed to pay attention to the SWG (the "red" light was on due to low salt, and I didn't notice it until I saw the all green algae in the pool) after I cleared up the green algae on the sides, I was left with the black spots on the bottom and steps.
    -Mike
    20K in-ground pool, Pentair FNS60Plus Filter, IntelliChlor, IntelliTouch, IntellipH, SAM/SAL, two WhisperFlow 1.0hp pumps, Water Falls, RayPak, Jandy

  6. #6
    swimdaddy is offline *Removed User* Weir Watcher swimdaddy 0
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    Default Re: Do I need to Shock to get rid of Black Algae - or is brushing enough?

    I was just in Miami for a week, beautiful, monumentally humid!

    Your CYA won't drop unless you 1)drain and refill 2) have a very big algae bloom (like what happens when we northerners winterize the pool and things go very bad) 3) you have a pool party for a LOT of kids and they splash half the pool water out (this one happens to me). Evaporation won't do it.

    That being said, 90 ppm is not bad, and with all the sun you get it may be a better that high.

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