I have been thinking about the topic lately after suspecting some stratification in my pool. Most of the time my pool gets multiple swim sessions daily (with many swimmers). Recently I purchased a 2 speed pump (see sig and thanks to those here who helped me pick it out) and started running the majority of the time on low. Last week we hit a two day stretch where weather and schedule meant no swimmers. I went out to measure my FC one night and got virtually 0. This freaked me out a bit...since I had not seen this before. I am using an intex swcg...so I went over to it and observed the bubbles coming off of the plate electrodes...a good sign it was working. I almost decided to supplement with bleach, but decided I would test one more day. I went out in the morning and my level was still very low...about 1. I was late for work, so I did nothing. That day the kids had friends over and also turned the pump on high (to use the solar panels...I have found the pump needs to be on high, or the panels are not as effective in heating). When I got home that night I tested and the levels were back up ~4 ppm. I am theorizing that the chlorine was sitting lower than I was drawing my samples due to lower flow and lack of disturbance. Also, I noticed since then that my return was point parallel and not down much, so I changed that. More testing needed to prove my theory.