I was out of chlorine reagent for four days so I was going by how red the water turned after adding DPD, as I had been testing it everyday and thought I had a handle on reading it. From the "Best Guess Chart" I need a minimum of 3ppm and it looked like I was within range. Obviously, I need a higher minimum since my zombie algae found a way to come back from the dead. Sigh.

It's more tricky than I thought trying to keep the chlorine level safe enough to use the solar cover yet high enough to prevent algae growth. Of course, I'm using the solar cover more for preventing evaporation than for heating the pool. I was surprised by how fast the water level can drop during the summers here, and the cover makes a big difference. It more than paid for itself as reflected in my water bill.