Is ur setup new? I've this problem often with my new setup. Once the tank stabilize it's over. But in my case brown algae occurs mostly in low light tank than hight light. I also have a 15 gallons with 55 watts compact x1, 20 watts fluorescent x1. Some algae appears but not a serious problems. I have upgrade from 3x 20watts. I just want to test how much amount of light my tank can handle and i'm changing fixtures and learning the dosing. My amazon swords grow 2 leaves a day now. i use two bottles diy co2 and change the solution in one weekly. I think 40 watts over 15 is not too high. And i agree with adding fast growing plants like hygrophila difformis or anacharis. Cut off the ferts and start dosing only when they shows signs of defiency. Good luck with ur tank.
