SLAM and OCLT questions

Aug 17, 2018
Milwaukee, Wi
After about a week of SLAMming the pool, things began to clear up, little by little. Yesterday the pool was very clear, so I did an OCLT.

The FC was 10.0 ppm at about 10:30PM. (I had not added any shock for several hours prior.)

At about 8:30 am before any sun hit the pool, the FC was at 12.0 ppm.

I've only had to do a SLAM once before (2018).

Why/how could my FC increase?​
Should I do another OCLT tonight?​
In the meantime, should I continue to SLAM? (CYA was 40, so I have been SLAMming to 16. Should I retest my CYA?)​
Thank you.


Gold Supporter
Jul 9, 2019
Winnipeg, Manitoba
I had this happen on two consecutive OCLTs - I’ve been sharing my journey in my Just Getting Started thread. I’m looking forward to hearing from the experts on this one.

Good Luck!



Mod Squad
Gold Supporter
Platinum Supporter
LifeTime Supporter
Jul 16, 2012
Central MD
FC reading doesn't increase unless you have a testing error, pucks in the pool, a SWG on, or you add it manually.

Yes, do another OCLT tonight.

Yes, continue to SLAM as you don't have a valid OCLT result.

CYA wouldn't have changed in that short of a period, but if you think your testing result may have been suspect then re-test.

Texas Splash

Mod Squad
TFP Expert
LifeTime Supporter
Agreed. And let's remember that during the SLAM Process it's important to maintain the proper SLAM level. Your CYA is 40 which means your FC should have been 16. Last night it was down to about 10 ppm. An OCLT can be done at that FC level, but letting it drop could impact the progress of the SLAM itself. I would agree .... maintain and do another OCLT. Good luck!