Chlorine loss 1-2 weeks after SLAM (twice in a row)

rhawke

Bronze Supporter
Nov 27, 2017
231
Houston, TX
Hi all,

About a month ago the water got a bit cloudy and I had OCLT of about 1ppm. The chlorine was below target level, so I decided to do a normal SLAM. Water was crystal clear already on the second day, OCLT showed 0 ppm loss. So I thought I was done. A few days later I did discover something that looked like pollen in one corner that I could easily brush off with my hand, only for it to reappear within 3-4 days, brushed it off again, appeared again.

Fast forward 2 weeks after the initial SLAM, Chlorine dropped below target level again. I figured that I had the SWG too low for the summer. So I did another SLAM. Same thing, pool cleared up, 0ppm overnight loss at the SLAM level. And to be on the safe side I increase the SWG output from 40% (2.2ppm) to 60 (3.3ppm), to make sure that this definitely wasn't the issue.

About 1 week later, the water got cloudy again and OLCT is again at around 1-1.5ppm and drops below target level overnight. The SWG is not able to produce enough during the day while it runs, so the water gets cloudier by the day. I did a few days of supplemental 3ppm liquid chlorine just to see if that helps, but that only slowed the cloudiness progress.

In addition my CYA level dropped 20ppm within 1 month. No draining, just regular splash out and a bunch of good rain showers in the past weeks. (overflow is closed)

Latest Test this morning after the overnight loss.
- FC: 2ppm
- CC: 0.5ppm
- TA: 80ppm
- CH: 400ppm
- CYA: 40ppm
- PH: 7.8


My two guesses:
- Mustard algae that is not killed by a regular SLAM?
- Does Mustard algae eat CYA? I heard some bacteria can? Or does a SLAM somehow reduce CYA? Or do I have two separate problems?

I want to make sure I get it right this time. Any recommendations?
 

Texas Splash

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Mustard algae and a SLAM does not eliminate CYA. The CYA either depleted a bit from normal oxidation, was a weak batch and dropped quicker than expected, or the testing variance of about 10 - 15 ppm was a result. Either way the CYA was a bit low (below 70) which placed more stress on the SWG and it couldn't keep up. At this point, I'd lower the pH to 7.2 and start a SLAM Process with an FC of 16. Leave the SGW off until after the SLAM is complete. Once you finish the SLAM, increase the CYA to 70 and that should hold you over until we hit the cooler months.
 
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rhawke

Bronze Supporter
Nov 27, 2017
231
Houston, TX
Mustard algae and a SLAM does not eliminate CYA. The CYA either depleted a bit from normal oxidation, was a weak batch and dropped quicker than expected, or the testing variance of about 10 - 15 ppm was a result. Either way the CYA was a bit low (below 70) which placed more stress on the SWG and it couldn't keep up. At this point, I'd lower the pH to 7.2 and start a SLAM Process with an FC of 16. Leave the SGW off until after the SLAM is complete. Once you finish the SLAM, increase the CYA to 70 and that should hold you over until we hit the cooler months.
That is pretty much exactly what I did the first time. I should have included that in the initial part. My CYA had dropped to 30ppm. I did the SLAM, waited for the OCLT of 0, then added enough per pool math to increase it to 70ppm and set the SWG to 2.2ppm which is what I usually needed in past summers. When I tested a few days later CYA registered somewhere between 60 and 70, pool was crystal clear, FC still around 11ppm after the SLAM.

I'm worried that if I just do another SLAM and add more CYA again, I will just be in the same position 3 weeks from now? I always have higher CYA loss than others on this website (around 10ppm per month) but this time was really high.
 

Texas Splash

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With no water exchange or leak, not sure why your CYA would fall quicker than most. But there have been times where some owners just get a bad batch of stabilizer. It happened to me one year. I'd add CYA and in a month or so it looked like I added nothing. Maybe next time try a different brand as a test?
 

rhawke

Bronze Supporter
Nov 27, 2017
231
Houston, TX
so I'm on my third day of the SLAM. I removed the lights for the first time and did find a ton of brown/yellow algae growing behind them. The water was already crystal clear yesterday, so I expected this morning to have no chlorine loss.

Instead it went from FC 16ppm to 11ppm overnight. However, CC was barely noticeable in color, so I would guess lower than 0.25. Is that normal? I thought if I have a lot of FC loss over night, I should get a lot of CC in the morning as nothing can burn off over night?

fyi on the CYA loss I just found this in another post from chemgeek ... so I guess doing 2 SLAMs in a row can account for the reduced CYA in 1 month.
Normally you shouldn't need to test the CYA very often -- monthly would be the most. The exception would be if the FC got to zero at any point in time, especially if this was followed by a high chlorine demand or significant CC, and after doing an extensive SLAM especially a yellow/mustard algae SLAM.

At least from a SLAM the lowering of CYA is not unexpected since as noted in Degradation of Cyanuric Acid (CYA), chlorine slowly oxidizes CYA. In pools at normal FC/CYA levels this is usually slow. In my pool it seems to be at around 2-3 ppm CYA per month but I have an opaque pool cover on the pool most of the time. For uncovered pools exposed to lots of sunlight, the loss rate may be greater from the hydroxyl radicals produced when chlorine breaks down so maybe gets closer to 5 ppm CYA per month (a few people report higher up to 10, but most don't without other explanations of water dilution).

A regular SLAM has 10 times the active chlorine level compared to regular FC/CYA levels while a yellow/mustard algae SLAM has 23 times the active chlorine level. So just based on this alone and using a conservative 2 ppm CYA per month base loss, a regular SLAM would lose about 5 ppm CYA per week while a yellow/mustard algae SLAM would lose 11 ppm CYA per week. If actual base loss rates were higher (closer to 5 ppm), then these SLAM rates would be proportionally higher so perhaps 10-15 ppm CYA per week for a regular SLAM and 20-30 ppm CYA per week for a yellow/mustard SLAM. If the pH is not sufficiently lowered before the SLAM begins, then the rise in pH from adding more hypochlorite chlorine will accelerate the CYA degradation since it goes faster at higher pH.
 
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Texas Splash

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Good catch behind the light. We see that often. It's possible the algae you released impacted your recent OCLT and perhaps it needs a full day or so to purge that live stuff from the water. You must be getting close. Keep it going!
 

Mdragger88

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Jun 1, 2018
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When you have completed the SLAM Process & raise cya back to 70 your target fc should be at least 5ppm. 3 is bare minimum- I suspect this is the root of your problem.
FC/CYA Chart.
 

rhawke

Bronze Supporter
Nov 27, 2017
231
Houston, TX
I had the target at 4ppm and it worked for a few days but I think the problem is also that we have toddlers frequently visit the pool and if a bunch of them pee in the pool the same day maybe I should manually add a bit of chlorine? I had gotten a bit lax with chlorine testing and only noticed the low FC after about a week.

Slam question: is the slam level the max or the min? In other words, should I add like 5ppm when the slam level is reached and watch it drift down back to slam level, or do I add up to the slam level, and 4 hours later top it off to reach slam level again? That part is not entirely clear in the slam article.
 

mknauss

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SLAM level FC is the maximum. Test the FC and add enough LC to get back to SLAM level FC.
 
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