Is this black algae? discolored grout

[Kijohn87]

Hi
I have a just slammed the pool after our old pump failed (which is now replaced). Pool is looking good and has passed the slam.
Having inspected the tile waterline a few areas are 'discolored' with slight blackish / grey marks. I have used a stainless wire brush during the slam process but no real change to surface.

Any ideas? Photo attached

Cheers
Ki

 

[willygee]

hmm..i am curious as well.

 

[Texas Splash]

Welcome to TFP! Looks more like an embedded stain than growing algae. Have you tried seeing if you can get part of a chlorine tablet to rub in that space to see if it's an organic stain? Hold it there for a few seconds? If nothing else seems to work, perhaps post a full set of test results so we can see all the numbers.

I also sent you a PM.

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[Kijohn87]

Thanks, yes have had a good go with a puck but doesn't change. There's a few small areas around the pool. Here's another area of the pool with the same issue - photo attached.

I'd be happy if it was a stain, just strange it's only in some discrete spots. About 5 areas spread around the water line tiles.

Also thanks for the PM!

Cheers

 

[Texas Splash]

There's also a chance it's a bit of organic material covered by scale. That can make a stain quite stubborn as well. It doesn't look like an iron or copper stain. If you wanted to be sure, you could rub a Vitamin C tablet on it (iron) or some dry acid (copper) to check. But I don't think it's metals.

How old is the pool/plaster?

 

[Kijohn87]

Thanks- unfortunately we don't have any vitamin c tablet or dry acid but will get some to check during the week.

The pool is 4 years old... I am not sure how long the stains have been there. I've only noticed them in the past few months when I've been up close inspect the waterline tiles and cleaning with bleach in a spray bottle.