Silicates in 0 tds water?

Hey guys, i've gotten conflicting answers from the google so I thought I would ask here. Can 0 tds rodi still have silicates? I've been battling diatoms since the tank was setup over a year ago and even if I scrub/siphon everything weekly with a toothbrush it just comes right back and is a thick dark brown coating over the rocks anywhere that the light hits.



If I flip a rock over its nice and white on the bottom, for a week or two before it gets covered so I'm assuming that I'm somehow introducing silicates into the system.



I just ordered a test kit for silicates, but I'm not sure how accurate a hobby kit for that will be.