What is the setup in the tank? If you have a good gap between your rock and the back of the tank, then 20" fixture is probably fine. If you are planning to use Kessil lights, then you are going to want the fixture mounted higher off the water anyway.
I watched the BRS adds (....oops, review) What I don't like is the T5's enclosed with no allowance for heat to escape from the fixture....either active or passive. T5HO's generate quit a bit of heat.
But if you don't have a canopy to hide a retro kit, then this is probably a nice option.