Post SLAM, High FC drop in a day

puppycrack

Bronze Supporter
May 10, 2017
21
Rochester, NY
Hi all. I am just coming off a SLAM process on my pool. While my chemistry wasn't bad, I was seeing more CC of 0.5 than I previously did. The water was also slightly cloudy, but I could see the main drains fine. I also noticed that I was vacuuming alot more debris than usual, as it was a fine yellow-ish silt that would sit in the liner seams. I've since discovered that it may have been mustard algae, as I also noticed a yellow tint in some corners just prior to SLAMing.

Water chem just before SLAM:
FC 5.5
CC 0.5
PH 7.8 (not sure why this was the case, bc 12 hours before this reading, PH was 7.4, w/ no chemicals added)
ALK 70
CYA 40

I'd note that my pool temp is also regularly 86-89F, and we cover the pool religiously.

I started the SLAM this past Monday, and have targeted FC to 16. I checked CL levels 2x per day (FAS/DPD test), adding CL as needed to reach target, and the FC never got below 10.5 (I was adding ~2 gal of 12.5% CL every day). Pump running at a lower speed (2,000 RPM) continuously. Vacuumed every day, and brushed walls once. I first passed the OCLT test over Wed eve/Thu morn, w/ CC at 0.5 on Thu morning. I took the solar cover off Thu morning, as it was covered up to that point. I kept the levels up for an additional day into today (Friday). I again passed the OCLT test last night, and my CC was at about 0.25.

This morning at 8am, my FC was 16.5, with CC at around 0.25. When I just checked now at 5:30p, and my FC is at 9.5, CC 0.25. We had the pool uncovered today, with a few hours of bather load (2 adults, 2 kids). UV was high, air temp got to about 86, and I heated the pool from 79 to 86. Pool is in direct sunlight from sun-up to about 6p. Does the CL drop of 7ppm seem high over 9.5 hours over this time period? Should I keep up the SLAM, even though my CC is <= 0.5 and I've passed the OCLT for two days now? I also see none of the silt I was seeing before, and no yellow stains. I don't want to stop the SLAM and have whatever was in my pool come back, but I also don't want to pour in CL needlessly either.

Thanks for any and all input!

- pc
 

jblizzle

Mod Squad
TFP Expert
LifeTime Supporter
May 19, 2010
43,255
Tucson, AZ
CYA protects a % of the FC from the sun. With a CYA of 40%, I would not be surprised if you lost over 50% of your FC in a day. This is normal when your FC is way above the maintenance levels. Passing the OCLT proves the losses were not from something growing in the water.
 

puppycrack

Bronze Supporter
May 10, 2017
21
Rochester, NY
CYA protects a % of the FC from the sun. With a CYA of 40%, I would not be surprised if you lost over 50% of your FC in a day. This is normal when your FC is way above the maintenance levels. Passing the OCLT proves the losses were not from something growing in the water.
Thanks for the input! This is only my second year owning a pool, and my first SLAM. I was concerned that such a high drop during the day was a problem, but so long as it's PERCENTAGE of drop, then when I get back to maintenance levels of 5-6, a 50% drop is only 1/2 a gallon or so of CL. On a great note, I continued to pass an OCLT last night, with FC dropping from 9.5 in the evening to 9.0 this morning, and a CC of ZERO! TFP is such a GREAT resource!!! Thank you!
 
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