I have a sand filter with 2 year old sand. Last year went well. This year, pulling the hair out of my head. We have not been able to use the pool as we would like. The water is crystal clear but alot of algae sitting on the floor. I have backwashed, rinsed, backwashed rinsed and same results. I brought in a sample and all tests were comfortably within range or slightly on the high side (Chlorine). After the last time backwashing, I placed a white sock over the jet and it was green. Why would the filter be sending algae back into the pool?