I cant stop brown diatoms from forming.. why?

[b_james]

My nitrates are near zero, all evap water and water changes are RO/DI with Hi-S. And I am always getting a lot of brown diatoms covering ONLY the sand. There is absolutly no algae eccept for coraline growing on rocks havent had any grow for the last 4 months. Every 3-4 days I get a light dusting of green on glass, bu thats it. Why is my sand a huge magnet for this gold/brown diatom. Its hard to see in the photo as the light is reflecting back, but the diatom is pretty dark.