You might check to see if your filter has enough sand in it. How old is it, any idea? You typically do lose sand over time with backwashing.

Let's also have you do the overnight test. This evening, test your water and add enough bleach to get back to shock level. Then, in the morning within two hours of sunrise, test chlorine again and see how much you lose overnight. Also, please test CC in the morning. Report these readings. If you are still losing more than 1ppm of chlorine overnight and have more than 0.5ppm of CC, then you are still fighting something in the water.

Then, tomorrow, we may see about turning the pump off for awhile to see if anything will settle to the floor that can be vacuumed to waste.