Nice thing about the stuff is that you don't need CO2 addition whatsoever, probably not even beneficial to have it since the duckweed can pull directly from the air.
Once it covers the surface, you may see it begin to receed after some weeks go by, often I see that from not providing enough nitrate/ammonia, water changes usually help perk things back up a bit too.
Cheers,
Raithan O. Ellis