I had experience with diatoms with my established tank. I was running top soil under silica sand and people were convinced it was my sand. But I tried many things to remove it, even purigen and it was temporary...
I noticed that in the shadded regions of the tank I didnt really get diatoms to show up, but in the areas where light was hitting I definitly got them. Tom kept saying its something with the co2 and light. So I bumped the co2 and noticed no changes. Then I decided to turn off one of my t5ho's...low and behold the diatoms started going away.
Till this day, I dont have any diatoms with my silica sand.
Is this brown algae?