Black algae or metal spots?

DrRadar

Member
Aug 17, 2017
5
Plano, TX
Just had our pool replastered with Diamond-Brite in April. We've been trying to keep pH at 7.2 per instructions but it likes to climb with the new plaster up to 8.0. (Today's test results below.) Yesterday, I noticed rust and black colored spots in the pool, particularly in creases. Some but not all of the spots would "smear" like they were oily but I wasn't really able to scrape any off and smear onto white paper. A nylon brush did nothing; a small metal pick did diminish but not eliminate the discoloration; a steel brush make a modest improvement but I don't know if that's too harsh on the plaster. It seems that the discoloration is largely in the pores of the plaster.

The deck was recoated after the pool plaster was done and considered that it may have been "over spray" of a chemical for that, but the clustering of the spots in creases doesn't support that theory (IMHO).

I'm inclined to conclude it's black algae but research suggests it may be due to metals. I tried putting a trichlor tablet on some of the spots but didn't see any change; maybe my poor technique or impatience. Didn't try the "vitamin C" test.

There are also a few patches of mustard/green algae, but not much. Still, an indication of inadequate chlorination.

The current measurements are below. The FC level does drop because high temperatures and long work days. Yesterday morning, it was 2.0.

Is 50 ppm CYA the correct target for north Texas considering the heat? We're using 2-3 gallons of 6% bleach a week. Is that typical?

I'm inclined to treat for black algae by superchlorinating and brushing. I guess I need an all-steel brush - correct? Are these available in 18"-24" widths for the end of the pole? Or narrower for more force? Any specific recommendations?

Thanks for any guidance,
Lloyd

pH 7.5
FC 6.5
CC 0.5
ALK 70
CH 400
CYA ~50

20200725 stains or algae.jpg
 

setsailsoon

Gold Supporter
LifeTime Supporter
TFP Guide
Oct 25, 2015
3,338
Stuart/FL
Dr,

Great close up photo.

If it doesn't lift with a brush or smear with paper it's not likely black algae. I would try the vitamin C test. Almost certain you have a metal issue if a trichlor puck doesn't affect it. Some of the spots even look a little rust reddish brown around the edges. When you do the vitamin C test you may find the stain redeposits a little way from the test spot. That happened to my stains and I had to do my first AA treatment that just finished up. If vitamin C works AA is extremely effective. My pool looks brand new again! Plus the AA treatment was very easy. I'm really surprised that the dosage was such a tiny amount yet extremely effective. Have you ever used an algaecide or mineral treatment of any kind?

If you have any algae I would want to SLAM first and get rid of it first. The AA treatment requires you take FC down to less than 2 ppm to begin and will reduce it to 0 very quickly. You can use Polyquat 60 algaecide (non metal) to prevent algae growth during treatment but it won't kill any algae present.

Your CYA level is correct and normal consumption of FC with no algae this time of year can be 2-4 ppm per day. I use about 3 here in S Florida.

I hope this is helpful

Chris
 

DrRadar

Member
Aug 17, 2017
5
Plano, TX
Very helpful, Chris. Thank you.

Since refilling the pool after the replaster, the only chemicals put in the water are a few pounds of stabilizer, bleach, muriatic acid, baking soda, and a few trichlor pucks occasionally (in the automatic chlorinator).

I cut a 1000mg vitamin tablet in half (to expose the inside) and rubbed it on a few spots. The stains were largely removed and the tile appeared whiter and smoother. But I didn't know if rubbing was legitimate...did the ascorbic acid or the friction remove the stains? So I crushed 10 x 1000mg tablets in a sock, dropped it on a spot, and covered with a plastic container to prevent dissipation. Then I went to hit a bucket of balls at the golf range. When I checked it two hours later, the spots appeared to have disappeared, replaced by a big smudge. Maybe the stain redeposited in a larger area or maybe other ingredients in the tablets caused the smudge.

Below are pictures of before, during, and after the vitamin C treatment. I included a corner and some tile as references. It does appear to me that the ascorbic acid did make a difference. High probability the spots are iron rust stains?

Never had a problem before and I don't know from where the iron may have come. The municipal water quality report (2019) indicates iron level is 0ppm, below the detectable level. Might it come from the new plaster itself?

Before vita C.JPGDuring vita C.JPGAfter vita C.JPG
 

setsailsoon

Gold Supporter
LifeTime Supporter
TFP Guide
Oct 25, 2015
3,338
Stuart/FL
Dr,

Looks a lot like my photos. Also looks like there was little less light in the last photo. I wouldn't expect you'd have iron from the plaster materials but who know? Sure looks like it is removed by AA. Have you talked with the people that did the work?

Chris
 

DrRadar

Member
Aug 17, 2017
5
Plano, TX
Very astute. That last picture was taken after dusk. No, I haven't talked to the company that did the replaster. I probably should but not looking forward to it. It was an aggravating experience. The Diamond Brite looks good but he dropped the ball on some things: Wrong tile, wrong drain covers for our cleaning system (although it was discussed beforehand), and more. Trusted he knew his business and was disappointed. I guess I'll suck it up and text him.
 

setsailsoon

Gold Supporter
LifeTime Supporter
TFP Guide
Oct 25, 2015
3,338
Stuart/FL
Dr,
Too bad about your experience with the company that did the work. I would run an OCLT just to verify it's not an algae problem. If you do an AA treatment you'll drop your FC levels very low... not good to do this with an algae issue. Even though you can uses Polyquat (like I did) it won't kill algae just inhibits growth. If you have algae I'd treat for that first. If you then have to go forward with an AA treatment you might want to read the post from my recent AA treatment here. Should give you a good idea of the process and help you gauge the chemical amounts.

Chris