OK. Thank you for your knowledge. I checked the sand filter as described and found no DE leakage and the pressure increased from 13 to 22 and then back down to 18 after a couple of minutes, so the sand filter is probably OK. I have used DE as an extra filter with the sand over the years and have had good results.

Last year, I kept the chlorine level high all summer and never had any bloom; this year, as I described, i have been hit with a slight green tinge and have been fighting it with >10 ppm Chlorine. Research articles explain that the Chlorine ppm should go up with CYA levels as you have explained but the article are difficult to understand even though I am a degreed Engineer! Their tables are very confusing unlike yours. In any event, with a measured CYA of 70 (according to Leslies' test), I probably need to be at around 30 ppm Chlorine to kill the algae.