Thanks Ben; that certainly makes sense. I am confused though, because I'm sure that some of the water I tested at some point was well below 10 ppm (no bleach added for 3-4 days and algae blooming back with a vengence) and the same water in the skimmer where a puck was sitting (pump off) registered about 3 ppm and today tested - no FC but about .5 to 1 ppm CC. Is there anything else that could be going on to knock the FC to zero so fast?