Black Algae? Am I on the right track?

fscott

Active member
Feb 19, 2010
25
San Diego, CA
Hi Everyone

So I think we have black algae but am not 100% sure.

The brief history is that we bought the house with the pool (pool is >15 years old), water was a little cloudy, got the TF-100 test kit and discovered CH >800 and CYA was >100. Bruce and Sal did RO on the pool water and get the chemicals to a manageable level (CYA was 0!).

In trying to get the CYA back up to 30, added stabilizer to 15 and then started using pucks + bleach. Probably let chlorine get too low a few times but are now being vigilant because....

Black spots have appeared in the deep end and they're starting to spread reasonably quickly! It was finally warm enough on the weekend to get in and take a closer look. They're round but don't feel slimy. I couldn't scratch any off to try the smear-on-white-business card test. They are very hard, same texture as the plaster. I've been scrubbing at the spots periodically over the last moth or so but they are definitely increasing in number and spreading. I'd say the area covered at this stage is over 3 sq feet and the spots are reasonably spread out. Besides the black spots the water is CRYSTAL CLEAR!!

So here's what I'm doing. Adding bleach 2x a day to bring FC up to 16, and holding there as best I can, scrubbing like a mad woman with a steel algae brush 3x a day.

Current levels are:
FC 16
CC 0
TA 90
CH 220
CYA 30
pH 7.6

Couple of questions:
1) Should I see CC with black algae?
2) Should I see FC loss with an overnight test with Black Algae?
3) Could the increase in number of spots be a consequence of me scrubbing the plaster TOO much? What color is the gunite underneath the plaster?
4) When I add the bleach I pour it directly over the area covered by the algae and let it sink to the bottom then scrub, then run the pump. Idea is that bleach will be REALLY concentrated on top of the algae. Is this a good idea or beyond stupid?
5) How long will it take before I see an improvement?

All advice will be much appreciated.

Will post photos ASAP.

Cheers,
Fiona
 

JasonLion

TFP Expert
Platinum Supporter
LifeTime Supporter
May 7, 2007
37,880
Silver Spring, MD
I don't think that is black algae. Metal stains are much more likely. Black algae is always slimy and will always scrape off at least a little bit. Metal stains are hard and smooth just like plaster.

Gunite would be a light gray. If you were seeing gunite, there would be obvious unevenness in the surface.


1) You don't always see CC.
2) You usually see an overnight FC loss with black algae, but not always.
3) No. If the only thing that could cause a problem with too much brushing would be crumbling plaster, which would be obvious.
4) Since these are probably metal stains, that is probably just making things worse.
5) Black algae will partially die back at high FC levels, reappearing in full force the moment you lower the FC level.
 

257WbyMag

TFP Expert
LifeTime Supporter
Feb 23, 2008
5,061
Denton, TX
You can check to see if they are metal stains by rubbing one of the spots with a vitamin C tablet. If they lighten up when doing so, it's likely metal.

On the flipside, if you scrape the spots with a trichlor tablet and they go away, then it's algae.
 

fscott

Active member
Feb 19, 2010
25
San Diego, CA
Ok, is it safe enough for me to jump in and try the puck test and vitamin C test with FC so high?

I'm a little suspicious of the metal deposit concept because the black spots only appeared after the pool was RO'd and FC got low. Unless of course I just couldn't see them when the water was cloudy BEFORE the RO.....

If it's metals, where are they coming from and how do I get rid of them?

Cheers,
Fiona
 

fscott

Active member
Feb 19, 2010
25
San Diego, CA
Ok I've tried a couple of things and are now more confused.

To test for Black Algae: tried my darndest to scrape off even a tiny bit of the black stuff and I get nothing. Then left a puck on a an area overnight that had three black spots - didn't do a thing!

To test for copper (metals) filled a nylon stocking with crushed Vitamin C tablets and left it on an area overnight with a couple of black spots. In the morning the WHOLE area covered by the stocking was BLACK!!

What the ****?

Overnight FC loss was 1. So no Black Algae? Yay....I think....

Under closer inspection there are a lot of little itty-bitty black spots all over the pool. And on the steps there is some reddy/brown loose precipitate type stuff that moves when I brush. Looks a little like shards of rust. I presume it's not only on the steps but that's the area I can see the best because it's closest to the surface!

So what's the verdict???

Cheers,
Fiona
 

fscott

Active member
Feb 19, 2010
25
San Diego, CA
Ok now I'm really really confused....if Reverse Osmosis removes metals (not 100% but most according to Jason the Site Administrator)...then how can my stains be caused by metals? The RO was only done 2 months ago...

Any thoughts Bruce?

Cheers,
Fiona
 

fscott

Active member
Feb 19, 2010
25
San Diego, CA
We bought the place in January and I haven't added any, but I can't say anything for the previous owners.

Is there a test I can do to see if an algaecide has been added?

Fiona
 

JasonLion

TFP Expert
Platinum Supporter
LifeTime Supporter
May 7, 2007
37,880
Silver Spring, MD
I keep seeing hints that it is either copper or silver in the water causing those stains. I was asking about algaecide because I was looking for a source of the metals I suspect are in your water. If you had used an algaecide with either copper or silver in it, we would have confirmation of my guess at what is going on and know how to treat the stains.

At this point, it might be worth trying a Jack's Stain ID Kit.
 

fscott

Active member
Feb 19, 2010
25
San Diego, CA
Coolio, I'll give that a try.

Another thing that might help with the diagnosis, I've been letting pH get up into the higher end (7.8) and then I bring it back to 7.4...should I be not letting it get so high and bringing it down lower? Therefore keep it between pH 7 and pH 7.5 kind of thing?