How much fake decor do you have in the tank? I had a really bad diatom problem (brown algea), which as stated above it from too much silica. Come to find out it was because of the fake rock I was using. Once I removed the fake rock, the diatom problem went away.
I have some fake decor, but it was used in my other tank before with no problem and the funny thing this Diatom algae is mostly around and on the live plants.