Brown Algae(?) In Established Tank

I have a 25 gallon tank that has been up and running for about 2 years. Pressurized CO2 , 2 T5HO bulbs about 8 inches above the top of the water, moderately planted, EI dosing.

Ever since the tank has been set up, I've noticed that my plants get cruddy looking fairly quickly. When I add new plants to the tank, they grow very well, but they seem to become covered with a layer of what looks like brown "dust" within a week or two. Its not really dust as it is not easily removed from the leaves. New growth on fast growing plants appears to be fine and then, again, within a week or so, it is brown and cruddy looking. I'll also get patches of brown "dust" growing on the glass near the substrate (flourite) very frequently.

Since the only brown algae I'd read about was diatoms and those are supposed to be a new tank issue, I'd just assumed that plants becoming cruddy looking in short order was just inherent to planted tanks. This weekend though I had the thought that maybe this is not normal behavior and that I have ongoing diatom issues due to high silicates in my tap water. It's embarrassing that it took 2 years to have the thought, but here I am.

I got a hold of 2 silicate tests and, with a healthy dose of skepticism, I've interpreted the results to mean that I have anywhere from 10 to 25 mg/l of silicate in my tank and tap water. I'm somewhat confident in these results as they're the result of numerous tests with both kits, one of which contained a 6gm/l reference solution. I know none of this is fool proof, but it's a best guess.

So, the questions I'd like to answer before I start trying to figure out how to remove silicates without removing phosphates (and avoiding RO/DI):
1) Is this cruddiness just normal behavior?
2) If not, are there any other brown, dusty looking algae to blame?
3) If not, does anyone have any reference for what constitutes high silicate values for a freshwater tank?


Dan