Black Algae Killed by pucks

[CTIGuy]

We have a 45K gallon Gunite Pool and thanks to this website and the test kit I have had an easy time keeping it clear and algae free for the past year. My pool is also a salt pool with a CL generator that runs at 100% (CL generator is undersized) for the 10-12 hours the filter pump runs and I still have to add bleach to keep it at the 1-3PPM CL suggested by the Mfg. Someone suggested I try adding more CYA as the Mfg suggests 50-75 PPM and I was at CYA 20 PPM and keeping the FC at 2 PPM. The CYA mfg suggested adding half of what was needed 1 day and the other half the next. With such a big pool I needed quite a bit of CYA so I added 1/3 the first day and another 1/3 the next. After a few days I noticed white crust lines on the walls near the sheer decent into the Spas and the spas tiles under the sheer decent were etched white. I assume this was the CYA but not sure so I asked the pool store and they said no way. My CYA now at 40 and not sure I want to add more and do more damage.

My other numbers
PH 7.6 (needs a lot of ACID to keep it there)
Alk 80
Cal 360

A few days after adding the CYA I started to see dark streaks on the bottom of the pool. Brushing did nothing except you could see a bit of a cloud coming off for a few strokes, I assumed I had been hit by the dreaded black algae.

Based on the advice read here I shocked (above 15 PPM), brushed and vacuumed 2-3x per day my pool for a week and it did nothing to the black algae. I brushed really hard on the algae areas but it seemed to help spread the algae. I had given up on the shock method and was going to wait till my kids go to camp in 2 weeks and try a “wet acid wash” on my Diamond Brite finish in hopes of bringing back the Midnight Blue color and getting rid of the black Algae.

Yesterday a neighbor asked me if I wanted his tri chlor tabs as he had the high Cya problems and has decided not to use them any longer. I had read here others had rubbed them on alga and it worked, so I took 2 and tried rubbing it on the streaks of black algae. After the rubbing and after playing with the kids in the pool for about an hour I noticed the streaks fading but it was dusk so I was not sure. This morning the algae is gone in the areas were I rubbed the pucks. I did not get the deep end very well as my puck was getting small and my kids wanted to play.

Lots of CL and brushing did nothing. I am sold on the pucks for black algae.

[chem geek]

In addition to using Trichlor pucks (which are highly acidic AND highly concentrated in chlorine) for getting rid of black algae, the other takeaway is a confirmation of what we've said all along about SWG systems. The superchlorination that appears to allow for lower Free Chlorine (FC) levels in keeping away free-floating green algae does not apply to algae stuck in biofilms on pool surfaces, such as black algae. So keeping the minimum chlorine level in Ben's Best Guess CYA chart (http://www.poolforum.com/pf2/showthread.php?t=365) is important. At 20 ppm CYA and 2 ppm FC that's at the minimum in Ben's chart, but at 40 ppm, you would need to have the FC at a minimum of 3, but probably closer to 4 ppm. This is the tradeoff between higher CYA protecting chlorine from sunlight vs. needing a higher FC level to keep away algae.

As for shocking, perhaps it needs an even higher level than that for green algae. There isn't a lot of experience with black algae on this forum -- mostly green algae and some yellow/mustard algae. Trichlor is the fastest way (and has been discussed as a definitive way to determine if a stain is organic or algae vs. a metal stain), but others have shocked and scraped with success so it's probably a matter of the degree of shocking. At any rate, you had success using Trichlor and that's what is most important. Now, at the 40 ppm CYA level you need to target an FC level of 4 ppm to keep away this algae.

Richard

[CTIGuy]

Richard

Do you think the white crust, etched Spas tiles and black algae had anthing to do with adding too much CYA at one time? I added 8lbs one day and another 8lbs the next. I was going to add another but stopped when I noticed the white crust and black algae.

If so do you have any suggestions as to how to add large ammounts of CYA without getting the damage or black algae?

On Cl level the Install and Ops guide from the Mfg of the CL Generator http://www.jandy-downloads.com/pdfs/H05675-RevE.pdf on page 18 says "Free Chlorine 1.0 - 3.0 PPM. Above 3.0 PPM may cause corrosion of
pool metals." Is this just more bad information from the Mfg? On the same page above "The AquaPure is warranted to operate properly
only if these conditions are met." Seems like a catch22?

[chem geek]

It is the ratio of the Free Chlorine (FC) and Cyanuric Acid (CYA) levels that determines the rate of corrosion from the chlorine. The SWG industry's own studies showed that (here (http://richardfalk.home.comcast.net/pool/Corrosion_and_Swimming_Pools.pdf)). The SWG manufacturers have always listed 1-3 ppm FC when that is clearly not correct. Most SWG users are able to run at 3 ppm FC with 60-80 ppm CYA, but not all. This does seem to keep away the free-floating green algae due to superchlorination in the cell (we think), but it does not good for algae stuck on pool surfaces.

I would not worry about corrosion of metal at the higher FC level so long as you have CYA in the pool. At least this particular vendor is disclosing the increased corrosion rates that can occur due to higher salt levels, but really the 3 ppm FC limit doesn't take into account CYA levels. Indoor pools without CYA will have a real corrosion problem even at 3 ppm FC as a couple of users found out with their stainless steel, but having CYA in the pool significantly reduces such corrosion rates.

So though I think your pool will be fine at Ben's chlorine/CYA table levels, I'm not sure what to suggest given their warranty requirements. It is a catch-22.

Richard