Check out brazilian pennywort. I'm just getting into planted myself and only have duckweed in one tank. I have another tank I'm about to get the pennywort for, and from what I see googling pics of the pennywort, it goes with the duckweed. Many tanks have both. The pennywort can be planted in substrate and grown up to the surface, or just grown on the surface un-rooted, and it does as well as duckweed in low-light conditions. Personally, I'd go with just the pennywort (as I plan to). You'll see it can grow fast and also isn't just for the surface, it can take over a corner easily and still have plenty of surface leaves....it's just a cool plant and I think it will be much easier to deal with then the duckweed.