We recently had a large algae bloom in our pool. We shocked it and ran the filter for three days straight. The pressure went up normally at first but then stopped increasing after backwash.
We took apart the filter and checked the grids, which are the original ones from the filter install in 2002. They were generally in good shape, but one had a small tear, so we ordered and installed new Unicel grids. We then loaded the filter with cellulose (11 5/8 oz, which is 15 scoops).
Running 12 hours with the new grids, the pressure is still not increasing from 13 psi (our normal pressure after cleaning). Also, when the spa is drained and refilled with filtered water, the water looks dirty.
We took it apart again to look for cracks/leaks. All parts look good, including O-rings. We're going to regrease the return tube (it had grease but we'll put on fresh). We will also now shock the pool again (not done for several days as we waited for the new grids to arrive). When we backwashed, the filter certainly had algae in it, but the return was not noticeably cleaner.
Other suggestions on what might cause the filter to return dirty water and have consistent low pressure with new grids?