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Thread: Getting more and more black spots on the bottom of the pool

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    Join Date
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    Getting more and more black spots on the bottom of the pool

    I have searched, read, looked, etc, and I can't find a definitive answer on what is happening to my pool. I even had a pool service company (highest rated one in the area on Yelp) come out and look, and he was stumped.

    We bought the house in September 14. We did a water change earlier this spring. I received my TF100 not too long ago, and I've been using the TFPC method, which has been great! We recently switched from pucks to liquid bleach as well.

    At first, I thought these were small specs of rust, as stated in my other thread. Some of the specs do, in fact, look like rust. I am having a hard time telling if they are all the same problem, if they go from redish/brown to black, or the other way around. At first, they were only on the bottom, but now I'm seeing them on the sides as well. They are pretty much on EVERY horizontal surface, including the top of the lights, steps, drains, etc., and now, they are even making their way onto the walls.

    I was thinking it might be black algae since it seems to be spreading somewhat quickly, but I can't even scrape it at all with my fingernail to smear it. I actually can't even really feel them at all, they don't protrude from the plaster.

    What I've done so far-

    Vitamin C Test - no change
    Held a puck on top of them - no change
    Tried taking them off with a stainless brush - can't
    Tried taking them off with pumice - some of them lighten up substantially, some don't at all

    Current #'s as of today-

    FC 9
    CC .5
    CYA 75
    CH 500
    ALK 160 (in the process of aeration to bring it down)
    Ph 7.2 (to lower ALK)
    TDS 1100 (from pool store)

    All of these pictures were taken this morning, before there was direct sunlight in the pool. One of the picture has my hand in it for size reference.











    Please help!!!

    - - - Updated - - -





    Las Vegas, NV
    12,000 plaster built in 09
    Pentair variable speed
    Pentair Clean and Clear cartridge
    TF100

  2. Back To Top    #2
    Mod Squad zea3's Avatar
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    Re: Getting more and more black spots on the bottom of the pool

    Unless you have a SWG 4.5 is too low for a CYA of 75. If you are using liquid bleach only then your FC range is 11 to 6ppm of FC. You should never let the FC drop below 6ppm.
    Try scraping some of one of the spots off with your fingernail and then smear it on a piece of white paper. If you see dark green it is algae. You should follow the SLAM process process and brush the spots with a wire brush daily. You should see improvement in a couple of days.

    Another question, is your plaster rough to the touch? Your CH is high and the pool is at risk for developing scale if it hasn't already. Algae like the spots you are seeing develop in the small crags in the plaster surface caused by the scale and that makes them harder to get rid of. To lower the risk of scale drop the pH to 7.0-7.2 and lower the TA to 70ppm. Use muriatic acid to drop the pH and TA.
    TFP Moderator
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    Re: Getting more and more black spots on the bottom of the pool

    Quote Originally Posted by zea3 View Post
    Unless you have a SWG 4.5 is too low for a CYA of 75. If you are using liquid bleach only then your FC range is 11 to 6ppm of FC. You should never let the FC drop below 6ppm.
    Try scraping some of one of the spots off with your fingernail and then smear it on a piece of white paper. If you see dark green it is algae. You should follow the SLAM process process and brush the spots with a wire brush daily. You should see improvement in a couple of days.

    Another question, is your plaster rough to the touch? Your CH is high and the pool is at risk for developing scale if it hasn't already. Algae like the spots you are seeing develop in the small crags in the plaster surface caused by the scale and that makes them harder to get rid of. To lower the risk of scale drop the pH to 7.0-7.2 and lower the TA to 70ppm. Use muriatic acid to drop the pH and TA.


    I really appreciate your input, but a lot of the stuff you asked me I covered in my first post to try and give everyone as much information as possible.


    The texture of our pool is smooth. It does not feel like any scale has built up to me. I honestly don't believe what I have is black algae, but I may be wrong.

    Thanks again.
    Las Vegas, NV
    12,000 plaster built in 09
    Pentair variable speed
    Pentair Clean and Clear cartridge
    TF100

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    Mod Squad kimkats's Avatar
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    Re: Getting more and more black spots on the bottom of the pool

    I am going to back you up some and ask..........

    Vit. C test----------how did you do it? Crush them up and put them in a sock on the spots? Or just rub one on a spot?

    Puck test-------how long did you leave it on the spot?

    Just trying to find SOMETHING to help you.

    That is such a pretty pool and setting

    Kim
    TFP Moderator 33x52 round AG 25,600 gals Sand Filter 1.5hp Pump - 2 Speed, SLAM, Pool School, Recommended Levels, Recommended Chemicals, Pool Math, Chlorine/CYA Chart, TF-100 Test Kit

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    Mod Squad kimkats's Avatar
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    Re: Getting more and more black spots on the bottom of the pool

    If you do not leave the puck on it for at least 15 mins you will not see any difference. It has to have time to do it's thing!

    Let us know how it goes.

    Kim
    TFP Moderator 33x52 round AG 25,600 gals Sand Filter 1.5hp Pump - 2 Speed, SLAM, Pool School, Recommended Levels, Recommended Chemicals, Pool Math, Chlorine/CYA Chart, TF-100 Test Kit

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    Re: Getting more and more black spots on the bottom of the pool

    Also, I apologize for putting my numbers down incorrectly earlier. I took my FC of 9 x .5, which is how I got 4.5. I had already multiplied my 18 drops by .5, so that puts my FC at 9.

    - - - Updated - - -

    Kim, thanks so much for all your help! The first time I did the puck test, I let it stay for 2 minutes. I just went out and tried 15 minutes, and none of the stains changed at all!

    For the Vit C test, I used crushed up pills, solid pills, and even some ascorbic acid from the pool store. I let them stay on for about 10 minutes, with no change.
    Las Vegas, NV
    12,000 plaster built in 09
    Pentair variable speed
    Pentair Clean and Clear cartridge
    TF100

  7. Back To Top    #7

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    Re: Getting more and more black spots on the bottom of the pool

    Anyone else?
    Las Vegas, NV
    12,000 plaster built in 09
    Pentair variable speed
    Pentair Clean and Clear cartridge
    TF100

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    cowboycasey's Avatar
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    Re: Getting more and more black spots on the bottom of the pool

    just throwing something out there...

    could it be something that was in the plaster and now coming out as you brush the pool? could you have a plaster person or company look at it, maybe they have seen something like this? how old is the plaster? 09?

    your pool is very clear but maybe try doing a SLAM for 5 days and see if the spots start going away.. it would not take much bleach to take it up to slam level and stay there..

    have you tried taking a spray bottle and do 1 part muriatic acid to 9 parts water and spray the spots to see if they disappear?
    Pool: Intex 16x32 15000 gal, 2 speed 340042, Pentair CC320 Filter, CircuPool SJ45 Salt System, Intermatic PE653RC; Hot Tub: 650 Gal SWG Megachlor
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    Mod Squad kimkats's Avatar
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    Re: Getting more and more black spots on the bottom of the pool

    I only wish one of them had worked.

    I am following what Casey said--------something bleeding through from time? Hummmmmmmm how to find a way to put MA on and keep it on the spots under water? A cup with a hole in it just big enough to put a turkey baster full of MA into it and squirt the MA while yo hold the cup in place.

    Kim
    TFP Moderator 33x52 round AG 25,600 gals Sand Filter 1.5hp Pump - 2 Speed, SLAM, Pool School, Recommended Levels, Recommended Chemicals, Pool Math, Chlorine/CYA Chart, TF-100 Test Kit

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