If you need to use a chemical means of ridding your tank of all types of algae, green water (free floating algae), etc then try algone.
I used this in a tank that was near a window and had free floating algae (green water). Nothing else was able to get rid of the condition. I tried massive water changes, other products, covering the tank, etc. I saw Algone.com and was a bit skeptical but figured I'd try it out. It clear the tank up within a couple days!