I'm pretty sure they don't make a 20k 36w PC bulb, but like Ray said, depending on your taste, the more blue look of your current setup might be fine. You'd probably see a little better growth if you went to 2 10k's 2 actinics. Personally, I like the look of the 2 & 2 and if I want to stare at a blue tank, I just flip the whites off...