I have an 8 year old pool that's running clear water with no sign of algae. My filter is a DE which I just replaced. I had this problem prior to replacing the filter. I went ahead and cleaned out the new filter with a hose and put it back together without DE this time. I run the pool pump and the pressure starts at 8 psi. After about an hour, it's up to 18 psi. I go ahead and backwash and the pressure is again at 8 psi, but after another hour, it's back to 18 psi. This is with no DE. Does anyone have a suggestion?