Sounds like you're there, 0.5 ppm is within the error of the 10ml sample test. Sounds like adding DE is the next logical step; be sure to check if it's blowing out the returns when you add it. BTW I haven't used the cellulose product you mentioned but I did try another last year and found that it didn't help and when I opened the filter I found it in large gray clumps on the sand.