It's a combination of high nitrates, Phosphates, light slow growing plants. Even if you get one of those things lowered, the other will feed the algae. Try to shoot for 10ppm of Nitrate, .5-.25 Phosphates and then all you need to do is limit the light and increase the water current to keep the leaves moving in the water, allegedly.
I'm not there yet, but this is what I've read and have been told. I only have surface movement and not deep current. Kind of leery about keeping a circulation pump on 12 hours a day. The fish love swimming the current though. I just don;t want to over do it for them. I also keep the light on 12 hours.