I've theorized that plants were actually the cause of my green water, Rockerpeller, so that might not work for everyone. The tank I had the problem on is heavily planted with 90w of lighting on it (3ft tank), whenever I tested it the nitrate levels were undetectable. I fertilize heavily and I figured that the plants need the iron & trace elements and whatnot to grow (which I was adding) but they also need more nitrates than were being produced by the fish, so they couldn't grow. But, and this is pure speculation but practice has borne me out on this, I thought the algae might be able to grow without nitrates, and just make use of those trace elements. I've put more fish in and have been feeding heavier than usual, and I think this helped the problem clear up - the nitrates still don't rise above zero though.