You are showing a more than 1.5 drop in FC overnight, which is a fail for the OCLT. Time to bring the pool back up to shock level and keep it there. Remember, the "M" in SLAM is maintain, you want to maintain shock level as much as possible.
I think you have something growing in the water. Didn't you have a 1.5 loss the other night on an OCLT? To go from 37 to 34 you would need to dilute about 8% of the total water in your pool, meaning an addition of 2,600 gallons.FC last night at 9:46pm: 37
This morning at 8:20am: 34
My chlorinator was off. We had some pretty heavy storms last night with lots of rain. Would that cause the chlorine to decrease? I know SLAM states no more than loss of 1ppm but I figure this is really close considering the amount of rain. The chlorine held steady all day yesterday in heavy sun with just the chlorinator running and no more liquid chlorine besides what I added the night before.
Jeremy
My opinion would be to SLAM the pool.
Once you pass that OCLT you know the pool is clean of the bad stuff.