Diatoms in a 9 month old tank

[Steven27]

Hello my fish tank has been set up for 9 months and I still have diatoms which I figured would have disappeared by now. My hang on the back filter runs Purigen, phosguard, and regular filter media. I have used distilled water in the last for water changes but have been using RODI from my lfs for water changes. The lights run for 6 hours and I feed once a day. What could still be causing the diatoms?

 

[Tuan’s Reef]

Have you tested your water yet? What CUC do you have in there?