A few weeks ago I bought two plants of Ceratophyllum demersum L. (hornsärv in swedish) at one of the local aquarium shops. I hadn't seen it before, and they told me it would grow fast and take its nutritions from the water, not having any roots.

I read a bit about it on the net as well, and according to some sources, it would fight for nutrition against the algae and probably win.

Boy, they were not kidding. It's growing 10-15 centimeters per week, and the amount of algae is constantly decreasing. Wow!