I had to hose down the cartridges Saturday morning AND evening, but since then the pressure has stayed very low. After a few days of 24/7 filter operation, I can finally see the bottom of the pool. It's not sparkling yet, but it's on its way. I'm glad we have a new, more energy efficient pump motor!

Lesson learned. Last night I had a vivid dream that we had one bad rainstorm and the pool went right back to green and slimy. But in the real world, it was a long term case of neglect.