We just opened our 24ft round above ground pool. We can not keep chlorine levels at a normal number. Water is crystal clear and ther is no visible algea in pool. Tested water last night at 9:00pm number were FC-2,Ph-7.5, Alk-125-130 Cal-150 and CYA 25-30. Tested again this morning and numbers were FC-0,Ph-7.5,Alk-130,Cal-150 and CYA-25-30. We use a Hayward S-166T high rate sand filter. The pool holds about 13500 gallons of water. I know that it is four feet deep with a vinyl liner. Can you tell me why chlorine levels are changing over night ?