You are thinking correctly in regards to the chlorine loss. The reason you are losing so much chlorine during the day is because of your CYA reading of 0. So, that can't tell us if you are still fighting something in the water. Only a chlorine loss overnight can give us that information.

It takes awhile to filter out dead algae especially when you have a larger than needed pump on an AG pool. In the case of pool pumps, bigger is NOT better. A too powerful pump forces debris through the sand bed instead of filtering it out very well.