Make sure you relieve the pressure, and drain the water from the filter housing prior to opening. Also, after backwashing a D.E. (diatomaceous earth) filter, one will need to add more D.E. back into the system. You’ll ruin the grids if you don’t add D.E.. Additionally, adding too little or too much D.E. can cause problems as well. Be sure you know how much you should add, and have a proper measuring device for measuring the added D.E..