Is this black algae?

[bkaiser]

New pool owner here. I have these spots scattered in the pool of the house we just bought. I scrubbed the one large spot pictured for about 5 mins with pumice and still it didn't come off. If it is black algae, what should I buy to help remove it besides a stainless brush? The pool was recently shocked before we purchased the house and is sitting at about 10PPM FC. Thanks!

 

[JamesW]

I don't think that it's black algae.

Possibly iron. Try vitamin C on a spot.

See if a magnet will stick to the spot.

 

[bkaiser]

I don't think that it's black algae.

Possibly iron. Try vitamin C on a spot.

See if a magnet will stick to the spot.

Just tried Vitamin C and it did nothing. Guess we know its black algae now. Anyone have tips to get it out of some very old, very porous plaster? I ended up having to resort to a pocket knife and dig it out of the plaster in one spot. It's very deep in there.

 

[mknauss]

Smear some of it on a paper towel. If black algae, it will smear a dark green.

Read Black Algae - Trouble Free Pool

 

[bkaiser]

Smear some of it on a paper towel. If black algae, it will smear a dark green.

Read Black Algae - Trouble Free Pool

Kind of an odd color, can’t tell. A little bit of red or brown in it it seems

 

[mknauss]

Most likely not black algae.
Those colors would imply iron.

 

[bkaiser]

Is it common for Iron to be so hard to remove? I let vitamin C sit on it for an hour with no results. Is there any other reliable way to test what this may be?

 

[mknauss]

It looks like a metal tie or rebar that is near the surface. As you say, you 'dug it out'. Did you crush up the Vitamin C?

 

[bkaiser]

It looks like a metal tie or rebar that is near the surface. As you say, you 'dug it out'. Did you crush up the Vitamin C?

When I dug it up there was only white plaster behind the stain, no metal thankfully. Yes crushed it up and let it sort of soak in.

 

[mknauss]

Hmm. Well, not sure what to say. You can treat it as black algae. Read the article I posted above. It mainly says to maintain an elevated FC for a long time and to disturb the areas with brushing, etc.

 

[bkaiser]

Read the guide thank you! I'm going to do that, the pool looks to not have been brushed in ages.