Advice on the black spots - black algae or stains or something else...

[yishen]

Hi there,
I just moved in a home with a very old pool a month ago. Has been learning how to maintain the pool by myself; balance the pool water, clean the pool etc... Got a lot information from this site and really appreciated!

The last thing bothering me is the little black spots in the pool. I noticed that after I clean the pool a month ago; there was a lot of the debris in the pool when I moved in; it's a foreclosed home.
I thought that's black algae in the very beginning. I tried to rub it with chlorine tab and took some sample with fingernail, but none of that succeed.
Then I tried to rub the spots with the vitamin C tabs; but the black spot still the same.

Last week I shock the pool and brush the black spots with stainless brush in the morning and evening, but still didn't see any changes. Have no clues what to do next.. I attached some photos I took underwater this weekend.

http://farm4.staticflickr.com/3874/14197115239_810c4636a4_b.jpg
http://farm6.staticflickr.com/5494/14197194680_c785962c02_b.jpg
http://farm4.staticflickr.com/3869/14403922083_b64e066bd2_b.jpg
http://farm4.staticflickr.com/3872/14382328552_ba6279d773_b.jpg

PH: 7.6
FC: 9 (test this morning; a week after I shock the pool)
TA: 70
Calcium Hardness: 200
CYA: 70

By the way, is above water safe to swim? My 2.5 years old wants to play in the water everyday. I stopped her for a week when I shock the pool.
Any advice will be much appreciate!

thanks!

 

[duraleigh]

That does not qappear to be black algae....it might be but it doesn't look quite right. Have you tried to scrape them off with a putty knife?

 

[JasonLion]

Try scraping off a little of a stained spot and rub the material on white paper. If you can scrape some off and if it smudges green on white paper then it is black algae. If nothing scrapes off it looks like it might be copper stains.

Swimming should be fine as long as FC is at least 5 and the water is clear.

 

[yishen]

Thanks for the advice. I will try to scrape it with the putty knife this weekend.
What it could be if not black algae? I also tried the vitamin C tabs, but no lucks.
The black spot looks like some small erosion on the plaster surface and black/brown stain inside.

I did a free chlorine test and it looks like no chlorine was consumed during the night.

 

[JasonLion]

Black can be either organic or copper staining if it isn't black algae.