on the pleated type filters. Mine were fine until last week when a new guy completely over gassed the pool to the point when I walked outside the gas nearly knocked me over. Anyway, the next day the pressure went up so high in the filter tank, not water was passing through. Never had that problem before. I have no idea what else he may have put in the pool, but after trying to clean the filters, the next day the pressure was back way up. I had to order new ones. $230 down the drain. Hope they arrive today, but am leery of installing them until I know what caused the sudden blockage. Also, it's a pain to change them.