i cleaned my pump out and replaced 2 grids in my filter system about 2 1/2 months ago. i noticed about 1 week ago my psi has been rising from 9 psi to 15 psi i dont think that is normal. i have been mainting all chemicals as needed. there is no d.e. coming out the jets so no other grids are damaged. how often do the grids need to be cleaned?