I have an in-ground pool with a DE filter. I had a very green pool this year when we opened it. I shocked it and then started the pump. It has been around 25 PSI, which is very high per our normal. I cleaned the filter but the pressure is still that high. Is this just because of how dirty the pool was or could there be something wrong with the filter or a leak? Backwashing seems to help for about 5 min then the pressure just goes right back up.