I use to have a pretty good problem with green water in my 72G discus tank. I was using a coralife T5 Dual bulb set with two Hagen Power GLO T5 18,000K (pinkish bulb) and a bare bottom tank. After about 1 day or so of about a 98% water change the green water started to return. I tried a couple of alge/green water products to try to get rid of the green water, but no matter what I did it still came back. So I decided to try a little experiment. I kept the tank lights off for a week to see what would happen. And gosh dammit no more green water. No I knew that I didn't want to keep the tank lights off cause I wanted to be able to enjoy the tank. So I ended up putting on the basic single fluorescent light fixture that came with the tank, and have had no more issues since. Heck I even stopped using any alge products...........