Thanks for the comment bro, its probably the dedicated lighting that fools you. The tank never gets direct lighting except for about 10-15 minutes when the sunset and its too low to be opposite my veranda and most days I have the shutter down so it doesn't even get that.I think you're trying too hard on the water aspect by going with RO. While you're trying (unsuccessfully) to completely remove all the silicates, you may be removing other trace minerals you'd actually like to have back where your plants can't get the upper edge on the diatoms. You could try switching to tap water (dechlorinated) for a couple weeks and see if that helps. Otherwise what you're doing sounds ok to me. From your picture, it looks like the tank is in a very bright location where you might be getting too much light what with the dedicated aquarium lighting combined with room ambient lighting.
I was under the idea that as long as you give the plants their minerals and the ferts they are ok. I really wouldn't want to switch to tap water. It's horrible! Way too hard, way too many silicates, I mean I don't even drink that water cause it taste so bad