Very fast growing floaters include Bladderwort, and hydrilla, (and other plants called waterweed) duckweed, hornwort, and stuff. Star duckweed grows slower than normal duckweed. I think star duckweed is an exquisite species. It lives under the surface rather than on top.
Waterweeds like egeria najas don't float very well but they don't HAVE to be rooted.
My tank sucks now
Fraternity of Dirt # 22