How do I deep clean my pool filter when backwashing isn't working?

Most likely, the filter media is fouled with oils, sunscreen, algae residue, or dissolved metals that a simple backwash can’t touch. For sand filters, use a degreaser or dishwasher detergent and let it soak overnight. For cartridge filters, soak the pleats in a chemical cleaner or replace them. For DE filters, take out the grids, soak them in acid or degreaser, and scrub gently. Also check the multiport valve gasket for leaks, but nine times out of ten it’s just a gunky filter.