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Thread: stubborn algae on bottom of pool

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    Join Date
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    stubborn algae on bottom of pool

    Hi all,
    I read the instructions on slamming the pool and followed it as best i can using bleach for shocking and working the filter 24/7.

    Before shocking, the water was clear, I only had algae on the walls and bottom of pool.
    Before shocking:
    Ph 7.2
    Cya : 60
    Fc: 2
    Cc: 0.5

    Using the calculator I added the necessary amount of bleach.
    In a couple days I was able to brush off The algae on the walls, also the patches at the bottom became smaller

    But then the improvement stopped, i still have algae stuck firmly to the bottom.





    After a week this is what is left in the deep end:





    There are also smaller patches in the shallow end, spa, and the steps.

    Also in the last 7 days the cya dropped lower and lower. Today it was less than 30 (thats as low as my test kit measures). The CC has dropped to zero for the last 3 days or so.
    But the Fc does keep dropping so I keep adding. Since the improvment stopped i raised the Fc to 'mustard algae' level.

    My guess is, since CC has been zero, the chlorine isn't killing anything. As for the cya I read in another thread the algae could turn it to amonnia maybe thats happening but i have only a small amount of algae so i am not sure. And i guess Fc is dropping due to sun light.

    I also bought a BBQ cleaning steel brush to see if the stuff can be scrubbed off, i couldnt get it off. I didnt try too hard in order to avoid damaging the plaster.

    Any advice? Should i try some other algaecide or buy a steel pool brush?

    Thanks.
    Last edited by Butterfly; 06-08-2014 at 07:26 AM. Reason: added img codes
    Concrete plaster. 14K gallons. Saltwater pool+spa. Hayward DE6020 filter. Hayward super II pump.

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    Patrick_B's Avatar
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    Re: stubborn algae on bottom of pool

    Welcome to the forum!

    May I ask if you have replaced a lot of water since this began? Trying to understand how the Cya would have been cut in half. Much of the time, that only happens with dilution.

    No, you do not need another brush, you just haven't completed the SLAM. If you still have living Algae, and it appears that you do, the simple reason is that there has not been enough FC in the pool long enough to kill it. Honestly, it is that simple.
    TFP Moderator
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    Butterfly's Avatar
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    Re: stubborn algae on bottom of pool

    Welcome to TFP

    Can you scrape off any of the black and smear it on a white card? If it smears green, it is algae.

    Actually, it kinda looks like black algae to me. Article in Pool School: http://www.troublefreepool.com/content/180-Black-Algae

    Brush recommendations:
    Nylon is for vinyl, fiberglass, tile, and plaster that is less than one year. (Plaster includes any exposed aggregate and marcite finish, btw!) Combo brushes are for older plaster. All stainless are for algae (very stubborn green algae and also the ONLY brush for black algae.)

    You CAN unscrew the Wall Whale from the nylon brush it comes with and screw it on a combo brush if your plaster is over a year old. Newer plaster is really too soft for anyting other than a nylon brush.
    TFP Moderator TF100 Test Kit TF100 TestKit YouTube Channel PoolMath
    You're done SLAMing when:
    1)You lose 1ppm or less FC overnight, & 2)You have .5ppm CC's or less, & 3)your water is clear.

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    duraleigh's Avatar
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    Re: stubborn algae on bottom of pool

    Welcome to the forum.
    I also bought a BBQ cleaning steel brush to see if the stuff can be scrubbed off, i couldnt get it off. I didnt try too hard in order to avoid damaging the plaster.
    Getting it identified may be a bit difficult. I believe you have calcium scale and then algae is embedded in it.....giving it that green tint.

    What is your current CH?
    Dave S.
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
    TFTestkits , PoolMath , Pool School

  5. Back To Top    #5

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    Re: stubborn algae on bottom of pool

    Thank you for all replies.
    I checked and saw Fc was the same as yesterday (9ppm)
    Did the cya test again, the dot was barely visible when i filled the whole tube so my guess is it is still20ppm or so.
    I added solid cya and sone chlorine. Then brushed the pool which didnt help much.

    Answers to your question on cya:
    I didnt dilute/replace the water, there was some rain but not much and the excess water evaporated.

    I didn't go in the pool so i couldnt scrape a sample from the thicker patches but i did try and scrape the stuff on the steps. It was very thin, not slimy at all. It felt as if there is no algae but just a discoloring of the plaster.

    I know the pictures dont look like it but this stuff is reddish brown.

    Is it possible that i killed the algae the first few days and am now left with discolored plaster?

    BTW heres how it started: when we bought the house the pool was in great condition.
    After one month of ignoring the pool, there were leaves and this stuff at the bottom of the pool.
    When we bought it, there was enough salt in the pool, chlorinator was set to 60% and filter was set to run 6 hours a day.
    I think at some point during the month the chlorinator shut itself off due to low salt. I believe this is how the issue got started. And while shocking i saw the message "chlorine level reached, cell turned off".

    Thanks again for your input.
    Concrete plaster. 14K gallons. Saltwater pool+spa. Hayward DE6020 filter. Hayward super II pump.

  6. Back To Top    #6

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    Re: stubborn algae on bottom of pool

    I tested the water again.
    CYA: 30
    FC:16
    CC:0
    PH: 8+ (necessary muriatic acid poured)
    TA: 140
    CH: 325

    Also as mentioned before, the stains are reddish-brown. I am going to try rubbing a cholorine tablet directly on the stains to see what happens. As I mentioned in previous post, I am not sure it is algae anymore. The CC is stable at zero.
    Any ideas are welcome.
    Concrete plaster. 14K gallons. Saltwater pool+spa. Hayward DE6020 filter. Hayward super II pump.

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