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Thread: Black algae and CYA

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    Black algae and CYA

    I have had a pool for 3 years and with the help of this forum have kept it clear. I was not diligent about testing this year and developed a black algae problem. I am now testing and have been shocking for the 3 days. There is a very slow change. My big question is that for the last 2 years my CYA was about 90. I did not drain the pool but was diligent about testing and so kept the FC level high. This year I did not test but basically put in the same amount liquid chlorine as last year but ran into this algae problem. When I tested the CYA was 0! How could the CYA go to 0 from 90 the year before. How can I bring the CYA level up?

    24500 gallon pool
    DE Pentair filter
    CYA 0
    FC 0 (just yesterday I put in enough chlorine to bring it up to 12)
    Ph 8.2
    TA 130
    CA 140
    Charlotte Area, NC
    24500 gallon gunite pool
    DE Pentair Plus Model 60 filter

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    Join Date
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    Re: Black algae and CYA

    Short answer because you have been on TFP since 2012 - failing to test may have allowed your FC to drop below minimum for your CYA, allowing algae to grow.
    CYA dropping to 0 also tends to point to an improperly maintained pool.

    CYA level can be brought up by putting stabiliser/conditioner in a sock and hanging it in front of a return.

    How are you "shocking" the pool?
    pH 8.2 is the maximum reading and your pH may be higher, I would advise you to adjust your pH first.

    Post back if you have any questions.

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    Re: Black algae and CYA

    Quote Originally Posted by Abigail K View Post
    Short answer because you have been on TFP since 2012 - failing to test may have allowed your FC to drop below minimum for your CYA, allowing algae to grow.
    CYA dropping to 0 also tends to point to an improperly maintained pool.

    CYA level can be brought up by putting stabiliser/conditioner in a sock and hanging it in front of a return.

    How are you "shocking" the pool?
    pH 8.2 is the maximum reading and your pH may be higher, I would advise you to adjust your pH first.

    Post back if you have any questions.
    1. Can I use dichlor instead of stabilizer to both shock and bring up CYA level?
    2. I have been maintaining the pool with bleach for the past few years. I get the shock amount by using the pool calculator and the CYA/chlorine index on the TFP website. I have been trying to keep the FC level at 12-15.
    3. The ph is moving all around. When I first started shocking 3 days ago the ph was 6.8 . Now it is 8.2. I do not understAnd how it moved up so fast. Given that it moved up quickly should I try to bring it down or wait to see if it stabilizes st 8.2
    4.i thought that CYA stayed in the pool until it was drained. I have not drained the pool at all. How could it have dropped from 90 to 0 even if I have not been diligent.


    Thanks for your time.
    Charlotte Area, NC
    24500 gallon gunite pool
    DE Pentair Plus Model 60 filter

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    Join Date
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    Re: Black algae and CYA

    1. Yes, as long as you understand what it does, and stop using it once your CYA gets in range.
    3. Bring pH down. Your TA contributes to the pH swing. We cannot recommend maintaining pH 8.2 as it is impossible to know if the pH is over 8.2, and our recommended maximum pH to maintain is 7.8.
    4. There is a bacteria that converts CYA to ammonia. Our understanding is that this is related to FC dropping below the minimum for the CYA, especially over winter. However, I am hesitant to suggest that this is what actually happened in your pool.

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    Re: Black algae and CYA

    According the pool school I can use muriatic acid to Lower the ph. Would you suggest that? You stated that I should lower the ph first . Also the article states that it can take days or weeks to lower the ph . Since I am battling black algae clearly I cannot wait that long. how long should I wait for the ph to fall?
    Charlotte Area, NC
    24500 gallon gunite pool
    DE Pentair Plus Model 60 filter

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    Smykowski's Avatar
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    Re: Black algae and CYA

    Lowering TA can take days or weeks.

    Lowering pH with muriatic acid will take a half hour, tops.
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    Re: Black algae and CYA

    Dichlor can lower ph, can I check the ph after I finish using the dichlor and then use muriatic acid as necessary?

    Thanks for your help
    Charlotte Area, NC
    24500 gallon gunite pool
    DE Pentair Plus Model 60 filter

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    Mod Squad jblizzle's Avatar
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    Re: Black algae and CYA

    Yes you can use the Dichlor to add the FC, CYA and lower the pH ... as long as you understand all the side effects.
    Jason, TFP Moderator
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    Re: Black algae and CYA

    Quote Originally Posted by Abigail K View Post
    4. There is a bacteria that converts CYA to ammonia. Our understanding is that this is related to FC dropping below the minimum for the CYA, especially over winter. However, I am hesitant to suggest that this is what actually happened in your pool.
    Not just an FC below the minimum, but near zero. It's easy to kill bacteria -- much easier than algae -- but of course once the FC gets below the minimum that prevents algae growth then if there are sufficient algae nutrients then algae can grow faster than chlorine can kill it. Once that happens, the FC may drop further close to zero and that's when bacteria can grow and consume the CYA. Sometimes this results in ammonia or partially degraded CYA which results in a huge chlorine demand while other times it results in nitrogen gas so there is no chlorine demand seen, just missing CYA.

    Regardless of how the CYA dropped, one really needs to test it at least at the start of each season since winter rains can dilute it. If the CYA gets to zero, then chlorine will get broken down by sunlight quickly, half lost every hour. If the chlorine stays low for an extended period of time, then black algae can grow -- it grows slowly so is usually only seen if the chlorine level is too low a lot of the time.
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
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