I seem to have a small amount if dirt/sand, possibly DE, that I just can't get cleaned up. To fine for the Polaris and the vaccum to pick up. I'm leaning more towards thinking it's DE....Why would I get DE in the Pool. Could it happen during backwashing? I have not seen any holes in the cartridge, atleast not seen by my naked eye . So if it's not DE and it is actually dirt/sand that is to fine to get vaccumed then how the heck do I get it out? (I do not use the filtering system to vaccum, I use water pressure...i.e. the polaris booster pump).