Inground, 20 x 40 vinyl pool, 20,000 gal. Hayward Super Pump, 1.0 HP, sand filter. System is 40+ years old! Filter pressure has been decreasing for some time to around 11PSI. I have (duh) routinely used tablets in skimmer in the past. Wonder if impeller could have been degraded what with age of system and my bad practices..........