Couple of weeks ago I had the pool company come and clean my DE Filter. It was full of DE, which explained why the pressure was building. They gave it a thorough cleaning. But lately the pressure has been building again. In addition I've been having algae issues, so I backwashed yesterday after cleaning the pool. All the green algae was released via the backwash, and then I added the recommended 6lbs of DE. Pressure was back to normal again, but within 2hrs or so it was back up to pre-backwash levels. Any ideas of what may be causing this? Thanks!