According to online paperwork the fluval edge uses 10W of halogen (unknown K spectrum) in their 6 gallon tanks. That's about 1.4 WPG right? (Even thought the write up says they give out 3WPG worth of light.) If the halogen gives off the right kind of light you should be able to grow plenty of stuff already. I know anacharis would grow, and probably make laps at the top of the tank.