OCLT Results at high FC (29)

zcmack

Member
Apr 1, 2021
18
Las Vegas, NV
Hello,

I have noticed yellow/brown build up on the sunny side walls of my pool and bermuda deck while doing my weekly brushing. I've also noticed increased chlorine demand to the point where I've been adding liquid to amend my SWG output. I failed OCLT Friday to Saturday, falling from 4.5 to 3 FC and ~.5 CC. I figured it was time to SLAM, however I was in a bit of a bad place as I had just raised my CYA to ~75 due to it being the hottest, sunniest part of the year. The pool appeared mostly clean and clear but the results suggest something hiding, I figured it would be an easy SLAM compared to some of the green pools I've seen on here.

I've been SLAMing at 29, brushing and vacuuming frequently. This morning I failed my OCLT from FC 29 to 24, despite having crystal clear water (I swear I could read a book at the bottom of the pool..) and 0 CC. Should I continue SLAMing today? Or is my high starting FC potentially skewing the results of my test? I had read some things on previous posts potentially suggesting this. I figured I would ask before I either ramp back up to 29 for another day or let it fall a bit in an attempt to get a more accurate OCLT result.

Thanks!
 

wireform

Silver Supporter
In The Industry
Aug 15, 2017
2,987
Spring Valley, NY
Yes, I'd make that 80 CYA and would check it again that it isn't higher. Slam for the appropriate CYA level and always round up to the nearest 10.
 

Texas Splash

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I would agree with WF. Keep on fighting the fight, but at the adjusted FC of 31. Be sure you have exhausted searching for ANY potential hiding places of algae. Leave nothing to chance even though the water appears to be very clear. Run another OCLT tomorrow and we'll reevaluate. If you do fail the next OCLT, post a couple pics of the pool so we can see your set-up just in case something catches our eye. FC testing at an elevated FC of 31 can be tricky and exhausting, so take your time and try to be consistent. Sounds like you are very close to wrapping this up.
 

wireform

Silver Supporter
In The Industry
Aug 15, 2017
2,987
Spring Valley, NY
so you're suggesting i should just adjust my SLAM FC to assume I am at 80 CYA, right? and keep on keepin' on?
Yes call it 80. As long as you're sure about the CYA you can slam on but if it were actually somewhat higher then the slam level you have would be ineffective and miss the mark on eradicating algae fast.
 

zcmack

Member
Apr 1, 2021
18
Las Vegas, NV
Held at 31 all of yesterday and scrubbed the inside of the skimmer, water fill well, behind niches and umbrella hole. Went to bed at 31.5 FC and just tested at 23.5 FC. This is a greater loss than any previous days test. Very discouraging. Any other tips or just keep bleaching? Filter pressure is normal.

Also I know pH is unreliable at high FC levels but should I be dosing at all during the SLAM? My pool tends to require about a gallon of muriatic acid every 7 days.
 

Texas Splash

Mod Squad
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Jun 22, 2014
33,434
Texas, San Antonio/Marion, South-Central Area
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Chlorine
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I wonder if all the cleaning yesterday kicked-up some junk that the chlorine used-up overnight? Can you post a couple pics of your pool so we can see everything you have there?
 

zcmack

Member
Apr 1, 2021
18
Las Vegas, NV
Hopefully these illustrate what you would want to see. I cleaned both lights in the pool and the single light in the spa. I currently have one of the lights out of the pool as I plan to replace the gasket on it today. Both niches were dirty but only one had a noticeable film on it. When circulating through the in-floor cleaning zones, the spa zone does overflow into the pool.

Other recent test results:
pH: 3.2 (prepared for SLAM)
Salt: 4450 (recommended level for my SWG is between 4200 and 4500)
CH: 325
TA: 160 (as per my intro thread, have not made adjustments to TA after I drained and refilled in April and it has been very slowly falling)
nightpool.jpgdaypool.jpg
 

Texas Splash

Mod Squad
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LifeTime Supporter
Jun 22, 2014
33,434
Texas, San Antonio/Marion, South-Central Area
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Chlorine
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Seems like a straight-forward build. Other than the lights you removed already, not many places for algae to hide. Under drain cover perhaps depending on age of your pool. When you did the skimmers did you check behind the weir door as well? Not auto-fill location? Curious to know if you see such a large FC drop on your next OCLT.
 

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zcmack

Member
Apr 1, 2021
18
Las Vegas, NV
Pool was built in 2005 and refinished in February 2020. I can pop the drain covers off and check I suppose. I scrubbed the entire skimmer assembly both the passage to the pool, the weir door/surrounding area, well and basket. The auto-fill well was pretty dirty/sandy but I put a few cups of chlorine directly into it and scrubbed the whole thing as well. For what its worth, I've been doing my final test at about 10pm and then checking again at 7:10am just before sun hits the pool directly.
 
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zcmack

Member
Apr 1, 2021
18
Las Vegas, NV
I remembered I have two waterfalls that I haven't used since May. They're hooked up to the old non-VSP pump that to the best of my understanding just draws from drains and recirculates to the pool (no feed through the filter). Just to rule those out I primed the pump with hose water, dumped some chlorine in and have those running too. Curious if hidden plumbing like that would be a candidate for algae? My understanding is that sunlight is required for algae growth.
 
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zcmack

Member
Apr 1, 2021
18
Las Vegas, NV
Well, two nights ago I finally had a less drastic OCLT loss and this morning I passed with a loss of .5! And I have just enough bleach leftover to clean my pool floats. Thanks for your help!
 
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Texas Splash

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Texas, San Antonio/Marion, South-Central Area
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Surface
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Salt Water Generator
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CircuPool RJ-45 Plus
full
 
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