A little background: I had excessive phosphates in my pool so I added a chemical to remove the phosphates. Instructions were to run the pool for 24 hours straight. The next morning I noticed that the pool pump sounded a bit different. My pressure gauge on my filter unit (cartridge type) showed only ~10 psi. Filter is immediately after the pump discharge (~ 3' of 2" pvc distance). Normally it is around 20 psi. I also measured the pressure at a spigot valve at the discharge and it also measured 10 psi. I opened the filter bleed valve and the flow was significantly less. I stopped the pump and took it apart (A whisperflo 2 hp single speed - WFE-8) to see if there was any debris inside. It was clean except for a little fibrous material at the diffusor (wear ring area - the wear ring did have some erosion but the impeller looked fine) which I cleaned out. I reassembled and the performance was unchanged. I checked the current draw for each leg of the pump motor and got 8 amps each so the motor looks OK, I think. I called Pentair and they recommended replacing the impeller and mechanical seal. The impeller looked OK to me but the pump was five years old so I decided to go ahead and do the replacement. That did not change the performance. There are no noticeable air bubbles in the strainer basket. My pump runs both my main pool and spa. I have two valves that I can switch and run just the spa or the pool. I also have a by-pass value which can take some pump discharge and run it into the spa which exits a small water feature - when I'm valved for pool use. The performance does not change no matter in which configuration I valve the piping. Sorry for the long story but want to lay out as much of the facts as possible. Even though I haven't proved it I'm thinking it is still related to the motor but would like someone else’s opinion. Thanks.