...after too many bouts with every kind of algae known to aquarium hobbiests, our T-5 SO lit 28 gallon tank displayed another problem: duck weed.
Our solution: Dwarf Water Lettuce (DWL). The DWL hasn't completely eliminated the Duck Weed, but it has eliminated 80-90% of the stuff. Dwarf Water Lettuce grows rapidly, and it makes for beautiful surface/top plant.
I forgive duckweed of all it's defaults. It's such a fast grower, when I find there are too much. It is easy remove with a net, but I always keep some. Duckweed is not great with hang on filter, that filter tends to push the duckweed direction bottom, but does great with a canister.
Dwarf Lettuce is great, but the roots can become cumbersome. I guess it is a matter of taste :-)