Black stains


New member
Feb 19, 2022

Can I ask for help, please? I have these black stains in the deep end of my pool.

January 10-19th, I had black stains that were slowly growing all along the line that separates the deep an shallow end.
I thought they were black algae and did a shock treatment plus scrubbing. Initially I used Calcium Hipochloride for the shock, but I threw it in the deep end, as a powder. I also threw in some Cloridize powder. Then I kept the chlorine high with liquid chlorine, tried to keep pH at 7.4 and kept scrubbing for over a week.
So now I have these 3 stains here, the ones showing on the picture (Jan 25th). This is different from before, it's not along the line like before, and it´s not growing anymore. I tried rubbing the trichlor puck on them, but it didn't help.
I read here that I should have dissolved the powders, and I might have made metals precipitate... oops.
Early February, I let the chlorine go back to 3-5FC. I tried a test with vitamin C in a sock and it made no difference. I also tried a test with Acid powder in a sock and saw no difference. I am not sure if I did the tests wrong. I had about 50grams of powder that I rubbed over a spot in the stain for about 2 minutes until it was all dissolved.

I'll write down the chemicals I measured then and now:
It's a tile pool, 35m3 or about 9000gal, temperature probably 81F (same as the ocean), dry season.
Feb 18th, with the 3 stains shown on the picture:
TA 100
CYA 50
Cl 5
pH 7.2-7.6 (I've been aerating it for a few months, bringing the TA down slowly, so pH is always changing)

Jan 18th when I had the black line growing
TA 180
CYA 50
Cl 1
pH 7.8-7.6

Also, my Cl test has expired, so I've been guessing Cl from the OTO test. It will probably be 3 weeks before I can get a new test shipped.

If anyone has some advice, I'd much appreciate. I don't know what to do next.
Thank you,


  • IMG_7083.jpg
    570.5 KB · Views: 43


Mod Squad
Gold Supporter
LifeTime Supporter
Jan 17, 2012
Evans, Georgia
Wow.....those are very dark stains! How do you normally chlorinate your water- what product? Do you read the ingredients on all the chemicals you add? I'm wondering if it isn't copper?

You need to update your test kit- pronto.

Remember that you need to keep your chlorine in line with how high your CYA is.
FC/CYA Chart

Do you have a pool store there that can test for metals?

Maddie :flower:
Thread Status
Hello , This is an inactive thread. Any new postings here are unlikely to be seen or responded to by other members. You will get much more visibility by Starting A New Thread