I read an announcement about closing the pool. It mentioned to slam 2-3 days before closing. It also stated that FC level will return to normal within a few days. I do not understand how the FC can drop to fast. What type of shock can drop the FC so fast? I ask because in my case normal FC is 6 PPM and shock level is 28 PPM. I cannot see how it will drop from 28 PPM to 6 PPM in a few days specially at closing time with the low temperature.