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Thread: Strange Black Spot on Bottom of Pool

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    Strange Black Spot on Bottom of Pool

    Last Friday I noticed a black spot on the bottom of my pool. It's about 3/4 inch in diameter. I've had black algae at the water line and in some poor circulation areas before, and it's clearly not algae. My pool water is crystal clear. I rubbed it with Vitamin C and it did nothing. I tried scrubbing the spot with a green Scotch brand scrubbing pad and it did nothing, except it seemed to scrape some of my plaster off. I can feel an indentation from the scrubbing. I didn't think the scrubbing pad was that abrasive to do that. The only thing I can think of is that an imperfection in the plaster was somehow revealed--perhaps by adding MA. This is around the area in the deep end (over a return) where I usually add MA.

    I will try to post a picture.

    My latest test results are:

    CL 9.0 (I'm lowering it to around 4-5)
    CC 0.0
    pH 7.8
    TA 70
    CH 575
    CYA 70
    Salt 3200
    Borates 50
    temp: 90
    CSI .08

    Last week I lowered my pH a little too much to 6.8 (I was lowering TA from 90) which caused my CSI to be about -.7 but it was at 7.2 (CSI -.38) a day later and a couple of days later it rose to 7.8 where I expect it to slowly increase to around 8.0 at which time I will lower the pH back down to around 7.5. I wonder if the 24-36 hours at such low pH could have caused the spot to appear or whether its from adding too much MA in one spot--my pool is only 4.5 feet deep in the deep end.
    9,200 gal. Gunite Luna Quartz French Grey pool with spill over spa, two fountains on tanning ledge (rarely used)
    Pentair Whisperflo 2HP, Pentair CCP Cartridge 420 Sq. Ft., Polaris 280 with booster pump
    Pentair EasyTouch 4, RayPak LoNox 266k Natural Gas heater, Aqua Rite SWCG T Cell 9, Borates, TF-100 test kit

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    Re: Strange Black Spot on Bottom of Pool

    See attached pic. I wish it was more close up, but it's very distinct.
    Attached Images Attached Images
    9,200 gal. Gunite Luna Quartz French Grey pool with spill over spa, two fountains on tanning ledge (rarely used)
    Pentair Whisperflo 2HP, Pentair CCP Cartridge 420 Sq. Ft., Polaris 280 with booster pump
    Pentair EasyTouch 4, RayPak LoNox 266k Natural Gas heater, Aqua Rite SWCG T Cell 9, Borates, TF-100 test kit

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    Re: Strange Black Spot on Bottom of Pool

    Furthermore, it's smooth to the touch. The black area is not rough or raised. It is within the plaster itself.
    9,200 gal. Gunite Luna Quartz French Grey pool with spill over spa, two fountains on tanning ledge (rarely used)
    Pentair Whisperflo 2HP, Pentair CCP Cartridge 420 Sq. Ft., Polaris 280 with booster pump
    Pentair EasyTouch 4, RayPak LoNox 266k Natural Gas heater, Aqua Rite SWCG T Cell 9, Borates, TF-100 test kit

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    Patrick_B's Avatar
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    Re: Strange Black Spot on Bottom of Pool

    I wish I could help, but I wanted to say way to go on posting results. You weren't leaving any chance for someone to ask for more.
    TFP Moderator
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    Re: Strange Black Spot on Bottom of Pool

    Quote Originally Posted by Brushpup
    I wish I could help, but I wanted to say way to go on posting results. You weren't leaving any chance for someone to ask for more.
    It also tells me that this is a real head scratcher. I have to believe it's a defect in the plaster job that was finally exposed after three years (almost) of wear and possibly from frequent MA additions over the area. Going forward I will trickle the MA around the pool a little at each return.
    9,200 gal. Gunite Luna Quartz French Grey pool with spill over spa, two fountains on tanning ledge (rarely used)
    Pentair Whisperflo 2HP, Pentair CCP Cartridge 420 Sq. Ft., Polaris 280 with booster pump
    Pentair EasyTouch 4, RayPak LoNox 266k Natural Gas heater, Aqua Rite SWCG T Cell 9, Borates, TF-100 test kit

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    Re: Strange Black Spot on Bottom of Pool

    Attached is a better photo. It shows the spot after scrubbing and the scrubbing revealed two small black dots near the bigger mark.
    Attached Images Attached Images
    9,200 gal. Gunite Luna Quartz French Grey pool with spill over spa, two fountains on tanning ledge (rarely used)
    Pentair Whisperflo 2HP, Pentair CCP Cartridge 420 Sq. Ft., Polaris 280 with booster pump
    Pentair EasyTouch 4, RayPak LoNox 266k Natural Gas heater, Aqua Rite SWCG T Cell 9, Borates, TF-100 test kit

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    Re: Strange Black Spot on Bottom of Pool

    I have a sopt that looks like that as well. I have even tried an acid wash, thinking it may have been some type of residue or oil. Nothing...didn't do anything. I have been suspicious that it is something to do with the plaster as well. It was done shortly before we bought the house...and it was a shady, no written warranty...kind of pool service that did the work. But I would also love to hear some ideas about what it could be.
    Brian, Richardson TX. 25K Plaster Pool, 1.5 HP Pump, Polaris 280 w/ 3/4 HP Booster Pump, 325 sq ft Cart Filter. This is my 3rd pool to convert to BBB method.

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    Re: Strange Black Spot on Bottom of Pool

    It may be the beginning of a tie-wire rust spot below the plaster surface bleeding through to the surface.
    If so, there would be a hint of brownish red color in the stained spot.

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    Re: Strange Black Spot on Bottom of Pool

    onBalance, thanks. I do not see any hinit of brownish red in the color. It's black and looks almost like a burn spot would look on carpet.
    9,200 gal. Gunite Luna Quartz French Grey pool with spill over spa, two fountains on tanning ledge (rarely used)
    Pentair Whisperflo 2HP, Pentair CCP Cartridge 420 Sq. Ft., Polaris 280 with booster pump
    Pentair EasyTouch 4, RayPak LoNox 266k Natural Gas heater, Aqua Rite SWCG T Cell 9, Borates, TF-100 test kit

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    Re: Strange Black Spot on Bottom of Pool

    Quit beating yourself up over your water chemistry treatment. Those spots have nothing to do with water chemistry.
    It may be that some organic debris (leaves, etc.) could have been troweled into the plaster mix and has now decayed enough to become visible now.

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    Re: Strange Black Spot on Bottom of Pool

    Wanted to report back on my black spot. It's now three black spots fairly close together and they are slowly growing. Even though they don't appear to be rust spots (more black than rust color) I'm thinking that's what they are.

    My question is whether and how to repair the plaster. The plaster is almost four years old. Other this one area, and except for the white spot etching described by onBalance in other threads, my plaster is in pretty good shape. I probably will not be able to get my plaster company to redo the job without a lawsuit. Any ideas?
    9,200 gal. Gunite Luna Quartz French Grey pool with spill over spa, two fountains on tanning ledge (rarely used)
    Pentair Whisperflo 2HP, Pentair CCP Cartridge 420 Sq. Ft., Polaris 280 with booster pump
    Pentair EasyTouch 4, RayPak LoNox 266k Natural Gas heater, Aqua Rite SWCG T Cell 9, Borates, TF-100 test kit

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    Re: Strange Black Spot on Bottom of Pool

    You are in Houston.

    I'd ask around. There have got to be several guys in the Houston area with good reputations for repairing rust stains etc. Go to or call several independent pool stores and get some recommendations. Then have one of them come by and give you an assessment and a repair estimate.

    I would also go to my pb or plasterer. 4 years is short, even with aggressive water chemistry for something like this to be showing up. No matter what your warranty says if you make a big enough stink they may come fix it for free.
    22k gallon IG pebblefina, Jandy 1.5 HP VS, Jandy CV Cartridge filter, Fafco solar panels, Polaris 360 supply side cleaner, waterfall

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    Re: Strange Black Spot on Bottom of Pool

    Could be black algae.

    http://www.troublefreepool.com/content/180-Black-Algae

    However you said earlier that after scrubbing you felt an indentation on the spot?
    Paul
    http://www.gastekservices.com A word of caution: When working with gas and electrical you might want to consider a licensed contractor. Consider the value of your life and others around you. If you would like to provide a review of the help I provided, please use the following link to leave a review. gastek - Google Search,

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    Re: Strange Black Spot on Bottom of Pool

    Thanks for the replies. It's not black algae. It's definitely coming up from below the plaster. I'm guessing it's not the plaster company's fault, but a poor gunite/re-bar job. My pb quit my job and I already sued him and I have a nice judgment that likely will never be paid.

    I'll ask around some of the local pool companies about plaster repair.
    9,200 gal. Gunite Luna Quartz French Grey pool with spill over spa, two fountains on tanning ledge (rarely used)
    Pentair Whisperflo 2HP, Pentair CCP Cartridge 420 Sq. Ft., Polaris 280 with booster pump
    Pentair EasyTouch 4, RayPak LoNox 266k Natural Gas heater, Aqua Rite SWCG T Cell 9, Borates, TF-100 test kit

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