Good observation. I get my water from my lfs, I help the owners a lot and can attest to the fact that the RODI units are well maintained. I use the same water in my other tanks that aren’t having this problem. Nitrates have been stable in this tank since the start, slight fluctuation in phosphates— I only have 3 fish in there.Looks like diatoms/diatom film to me and diatoms not only appear in new tanks but mature tanks as well. Diatoms feed mainly off of silicates but also thrive off of dissolved organic compounds, phosphate and nitrates and silicates. Tap water can emit silicates andcause a bloom if you use it to mix salt or to replace water that evaporated from the tank. Tou can also get a diatom bloom when using RODI if the cartridge which removes silicates expires.
Any other ideas though??
I even tried blackout vinyl on the side of the tank facing the window…