Black Algae??

[FourA]

I am hoping for some help in determining if this is black algae or something else?

I have been dealing with this issue for a few months now. These black stains began appearing around a few of my pool returns. It is not all, just 2-3. When you rub your hand over the black spots some of the black rubs away and you can see a little gray/black on you finger. It is little smooth/slimy at first touch but doesn't exactly a slick as normal algae.

I have used wire brush to get remain black off and it general disappears after a few passes. I have tried rubbing chlorine pucks over after wire brush and the black comes back within days. I have also shocked several times up to 35 PPM and have maintained the chlorine higher at about 12-13 PPM however this black keeps reforming if I don't brush in the those spots every day with a wire brush. The rest of the pool looks great.

Here is what I have:

Pool 1.5 years old
Pentair SWG
Alkalinity: 50
PH:7.2
Chlorine:21 (Shocked last night)
Salt:3600
CYA:40

Any ideas what this is? Is it black algae or something else? The pool guys say to use Copper Algaecide however based on my readings on this site that does not sound wise. Here is a picture of it. I appreciate your help.

 

[jblizzle]

Does not look like algae to me, but if you scrape some off with your finger nail and spread it on some paper, it would look green if it was algae.

What color is the pool supposed to be? Are those cracks everywhere? How old is the plaster?

 

[FourA]

Thanks,

I don't know if I can get enough on my finger to transport but try it. I will have to wait a few days for it to build up enough to give it a try. If it is not algae any ideas on what it could be?

Those are cracks. They have been there since the pool was built 1.5 years ago. The builder said they are not structural so he will not do anything with them.

- - - Updated - - -

I forgot to answer your other question. It is a marble grey/blue plaster. Thanks

 

[jblizzle]

Wow, that was a really bad builder then. You should have gotten it replastered in my mind.

I have no idea what it could be. But must be something seeping from behind