Welcome!
I'm sure the other guys will chime in with other suggestions about duckweed, but I saw this the other day:
http://www.youtube.com/watch?v=acl2qO0QTDo
http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=226808

Might be an option, I'm not sure how much to buy or rent though


Central OK: 2 Acres w/ LMB, GSF, RES, CC