I've had good luck with azolla. While it can cover the tank it is easy to thin
out and grows reasonably slow.

Moon
remove nospam from e-mail to send to me, I grow trees in aquariums like bonsai.
I breed dwarf crayfish, great for planted community tanks. If you can get me a
shovelnose sturgeon fingerling (Scaphirhynchus platorynchus) no wild caught
please, contact me