Is this... black algae?

[peacefulkancer]

For this season I have something black on my [horizontal] walls. I've SLAMed twice and done the OCLT twice which came back 0ppm and 1ppm respectively. I wire brush these areas at least once a day and nothing ever changes. The areas have probably doubled since February, when I first noticed them. The filter has been cleaned and recharged.

Last Test:
FC: 7.5
CC: 0
TA: 110
CH: 400
pH: 8
CYA: 45
NaCl: 3000

I know the CH is high but this is pretty much what comes out of the tap here in Phoenix, AZ. Area is not raised or slimy - feels like rest of pebbletec. Almost seems like it is the plaster, which is normally a brownish color.

On bottom step in corner with a little up on second step:

 

[JamesW]

Did the filter have any turquoise or gray stains?

It might be copper or silver.

Have you used copper or silver algaecide?

Drop some vitamin C on it to see what happens.

 

[peacefulkancer]

Did the filter have any turquoise or gray stains?

It might be copper or silver.

Have you used copper or silver algaecide?

Drop some vitamin C on it to see what happens.

Turquoise - I would say no. It was mostly the normal brownish sand with a tinge of green like it always is. This water is only about 2 years old and I've never used any silver, copper, or any other algaecide in this water. I'm not sure how I would drop Vitamin C on it as it is under water. Help?

 

[JamesW]

Get some vitamin C tabs and drop them on a spot.

 

[YippeeSkippy]

Cheap Vit C tablets in a sock.....bash it a bit to crush a few up.. .lay sock on stain for a few minutes and see if that lifts the stain or changes it in any way. Report back.

 

[peacefulkancer]

Get some vitamin C tabs and drop them on a spot.

Cheap Vit C tablets in a sock.....bash it a bit to crush a few up.. .lay sock on stain for a few minutes and see if that lifts the stain or changes it in any way. Report back.

I put a dichlor tablet on one spot for ~3 mins. No effect.
I put crushed Vitamin C tablets on a spot and left for ~15 mins (until they 100% dissolved). No effect.

 

[JamesW]

Not sure what you have going on.

 

[peacefulkancer]

What effects are we looking for with Vit-C?

 

[JamesW]

Vitamin C might make a clear spot or a darker spot. Mostly looking for any reaction or difference.

The stain might be mottling.

Gray Mottling Plaster Discoloration

Gray (also spelled “grey”) or darkened blotchy or streaky discoloration of pool plaster (cement flatwork) is known to be caused by several improper plastering practices, including the addition of calcium chloride to the mix (even less than 2%), late hard or dry troweling, and sometimes a cement...

www.troublefreepool.com

 

[peacefulkancer]

Vitamin C might make a clear spot or a darker spot. Mostly looking for any reaction or difference.

The stain might be mottling.

Gray Mottling Plaster Discoloration

Gray (also spelled “grey”) or darkened blotchy or streaky discoloration of pool plaster (cement flatwork) is known to be caused by several improper plastering practices, including the addition of calcium chloride to the mix (even less than 2%), late hard or dry troweling, and sometimes a cement...

www.troublefreepool.com

Interesting with the mottling. I wonder why it is suddenly showing up and seemingly "spreading."

 

[peacefulkancer]

Has anyone ever tried pressure washing with water still in the pool? The water would reduce the psi seen on the pebbletec. Thoughts?

 

[peacefulkancer]

So a few things... I tried pressure washing and nothing came off. Secondly, I went in and inspected it up close with goggles. It looks like it is the non-rock part (the aggregate?) that is turning from brown to black. Is this mottling?