I don't know if my experience will be of any help to you, but it's something to consider to see if you have a similar situation. I have no acceptable solution, though.
I have always had diatoms in my tank. A 4+ year old tank and still diatoms are an issue. My diamondback gobi keeps my substrate nice and clean by constantly turning it over. My float mag takes care of my glass. I have to use it 3 times a week and it is always diatoms I clean off, not green algae. My rocks are usually pretty clean, but if I add a new rock, even a baseball sized one, I get a big outbreak of diatoms on it and some on other rocks. I added some new rock work 9 months ago and am still suffering with lots of diatoms, even on the old rocks. The new rocks are very brown. It does brush off with a toothbrush, but I can't clean the rocks with a toothbrush 3 times a week so I just leave it.
I use GFO, helps reduce diatoms very very little, I have a refuge with macro algae. No substrate in the refuge.
I think my issue was a 2 year overdose of biopellets during the first 2 years of the tank. Once I stopped them, my diatoms eventually "almost" disappeared. My glass even started growing green algae instead of diatoms. But my corals stopped growing (carbon limited, yes they need more carbon than just CO2), and my nitrate began to climb like mad. I switched to other forms of carbon dosing. Corals started growing again and nitrates are back down to decent levels, but diatoms are an issue again. I have tried dosing with alcohol, vinegar, glucose, and combinations of the three. Diatoms seem to not be finicky about their carbon source.
I use RO/RI water and have even tested Si. The test kit reported 0 Si.
I personally think my tank has hit a balance like this because of prolonged overdosing biopellets, and now I need to do something drastic to shift the native inhabitants away from diatoms to green stuff permanently. Just not sure what that may be because being carbon limited is not the answer because I love to see my coral grow.
I have also done bacterial dosing to try to get strains established that will outcompete the diatoms. No success there.
Now I have to admit the red light issue could be something to consider. I run my red lights at the same strength as my whites and have done so since day one. I run my whites close to what I run my blues at, and again this has been consistent since day one. My lighting is AI Hydra 52 HDs. According to the AI app, my peak lighting is at 12,000K. My peak lighting period is 3 hours. Outside of that blue:white ratio is higher, though. Red:White ratio is always 1:1. I will back off my reds to 50% of my whites. I am not going to change my whites at this time. I will see how this works.
