Join Date: Dec 2012
Location: Champaign, Illinois
I'm a big fan of salvinia. It's structurally similar to duckweed but larger and slower growing. The size makes it easier to manage (move, remove, get off your arms, not transfer to other tanks... et al :))
Water sprite (Ceratopteris thalictroides) will also do a good job growing on the surface.
I haven't had luck getting my java moss to float, but I had mine growing near my water sprite and the two joined forces to form this giant floating island.