Black spots

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N

nlindelldc

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Mar 21, 2015
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Corpus Christi, Texas
So the other day I noticed some very dark (seems black) spots in the deep end of my pool. They appear about the size of nickels I would say and there are about 12-15 of them all grouped in an area about 2 square feet or so. I know you will all ask, so I will give you my readings. I just finished a SLAM for a small algae bloom that was just starting so my FC numbers are high but coming down. CYA 80, FC 25, PH 6.8 CC0

The water is getting a bit chilly just now (temp was right at 80 yesterday) so I sent my teenage son into the pool armed with a chlorine puck. No luck, he said it wouldn't come off. So next I sent him in with some Vitamin C tablets and again, no luck. I then sent him in with a wire brush (told him to try to be as gentle as possible and to not scratch fiberglass finish if possilble). None of it is working! I'll try to take a pic later when I get a chance but this one has me stumped
 
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Calcium hypochlorite is a granular chlorine. If you broadcast it into the pool, some granules might have reacted to copper in the water and created the black spots.

Is there a lot of copper in the water?
 
JamesW said:
Calcium hypochlorite is a granular chlorine. If you broadcast it into the pool, some granules might have reacted to copper in the water and created the black spots.

Is there a lot of copper in the water?
Click to expand...

I have never used Calcium hypochlorite. I have no idea about Copper either but I am on City water, not well water so I wouldn't think it would be an issue. So are you still suggesting I try the Calcium Hypochlorite in a sock?
 
I'm thinking that calcium hypochlorite might have caused the problem, not that it might be a possible solution.

Do you have pictures of the spots?

How fast did the spots develop?
 
JamesW said:
I'm thinking that calcium hypochlorite might have caused the problem, not that it might be a possible solution.

Do you have pictures of the spots?

How fast did the spots develop?
Click to expand...


I have never used Calcium Hypochlorite in the pool and I am the original owner. I will try to post pictures in a bit. The spots seemed to have developed overnight. I also have not tested CH in a long while, I will do that this afternoon as well
 
Okay, I was able to snap a few pics. I realize now that my oriignal estimation of size was too large, they appear to be more the size of peas.
Full tests at noon today were:
FC17, CC1, PH7.6, CH0, TA100, CYA 70
 

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So a little update.....my son went to work on the spots again with a metal brush. He said that when he hit them with the brush, the black spots appeared to turn grey, like he had covered them with a pencil. Also, the spots appear flush and not raised on the surface. I'm leaning towards some type of metal stain at this point but will try the Vit. C and see if it works. Is there anything else to try for metal staining? These appear pretty stubborn
 
Fertilizer with Iron? My bermuda loves it some iron, but you have to be really careful to only get it in the grass as it will leave rust spots on my driveway. I imagine it would add similar spots to the pool surface.
 
Just to answer all of the above questions, we had a ton of roofing jobs in our neighborhood about 2 years ago but nothing recent. No metal being cut anywhere near the pool. I do fertilize my lawn twice yearly with fertilizer that I'm sure has iron in it. But I am super careful to not get it too near the pool. I haven't fertilized since the
Spring and these spots have only been around a couple of months
 
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