Please help me to diagnose my water circulation issue. I have a 1 hp single speed pump that goes through a 38 sf Anthony Apollo DE Grid Filter. Up until now the whole system was pumping fine with good suction at the skimmer and a good flow at the returns. Yesterday I backwashed the filter prior to adding CYA to the pool and when I was finished the pressure was 12 psi at the filter (which is normal after backwashing). I ran the pump all last night and all day today to let the CYA dissolve and when I checked the pool after work today at 5:30 PM, the system seemed to be running fine and the pressure at the filter was 10 psi. At 6:00 PM, the system shut off automatically as it was programmed to do. I turned the pump on manually to allow more time for the CYA to dissolve and the pressure at the filter was 0 psi. You could hear water moving through the pipes, but it was moving significantly slower than normal. The flow at the returns was approximately Â¼ as strong as normal. Is this a pump issue, or is there a blockage in one of the lines?