Black algae removal

[roberte37]

Hi there,
Have had dark black streaks forming in my infinity basin for a while now and wasnt sure what it was for a while. The rest of the pool is fine. I think I wasn't keeping my CYA and Chorine high enough per the recommendations of the site for a SWG so I started doing that. The basin doesn't get the circulation that the rest of the pool has so I have been running the booster pump more to create more return of that water. I recently did a slam of just the basin because my CYA is up at around 80 -90 and I didn't want to add some much liquid chlorine to the whole pool. I closed off the return of the basin and had lowered the water level in the basin till I got my CYA down around 60 and then I brought my Chlorine up to around 24. in just the basin. I measured it again after the sun went down and of course it had dropped so I added some chlorine to bring back up and let it just sit in the basin overnight. I scrubbed with a stainless brush a lot the next day but it only really dulls the black and seems to have smudged the surface more. The slam didn't remove much it seems. I bought a trichlor puck and am ready to try that. wasn't sure how to do that though? How do you avoid getting it on your skin.? and I would have to be standing in the infinity basin which is about 650 -700 gallons.. I have seen some threads talking about using a sock? to hold it? I've attached a couple pics, the shadow are me taking the shot

 

[JJ_Tex]

I would hold off on the wire brush for now. Just based on those pictures, it almost looks like you have brushed off all of the plaster.

I would read up on Black Algae in pool school, especially the part identifying it as black algae including the part I marked in red:

However, there are a couple of black algae “false-alarms” if the black spots appear as isolated dots on the pool surface. The first of these can result if a granular fertilizer or lawn treatment containing iron was applied anywhere near the pool. If any wayward granules fell into the pool, the iron in these granules will cause dark spotting where they landed. The second false-alarm is that smooth white plaster surfaces sometimes contain small dark aggregate particles embedded within the plaster (usually near steps) that act as an anti-slip surface. ​

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If you have ruled out the false-alarms described above, then gently scrape a sample of the suspected black algae with your fingernail and then smudge it on plain white paper. If the smudge scrapes off and is a greenish color, then it’s black algae. The good news is that it can be removed.​

Black Algae

This guide provides a step by step procedure for eliminating black algae in your concrete or gunite pool. Read full post here.

www.troublefreepool.com

 

[JamesW]

It looks like paint.

If you rub your hand on it does it leave white paint on your hand?

 

[roberte37]

I did read the article on the TFP site. I am pretty certain it is not aggregate pebbles. I just now went into the basin and scratched with my finger. Nothing scraped of. It doesn't leave white paint on my hand, but when I scrub the bottom it gets a little hazy from the surface being abraded. I'm starting to think it is not black alage now. The rest of the pool is Quartscape but I thought the infinity basin was plaster. Is it possible it is paint? When they were finishing building it I saw the worker painting on some dark waterproof looking like material in there. I don't what they put on to cover that turned it light colored. I assumed it would have been plaster. Is it common in pool construction to paint infinity basin bottoms? How does that hold up to the chemicals? in pool water.
Mostly, I've brushed with a nylon/stainless brush over the past few years to brush the small amount of green algae that has grown in there which comes off easily. I would be relieved if its not black algae. Its just kind of unsightly then. I have to do some more information gathering I guess on how that infinity basin surface was finished off.