Hi
I would recommend Salvinia natans. Check out the Tropica site for some more details. This is an easy to control floating plant due to it's size. Unlike duckweed, which is an absolute nightmare to have in your tank, this is a floating fern which is relatively large (each individual leaf can get up to a couple of cm in optimium conditions-usually ~1 cm wide though). While it grows quite quickly, it isn't anywhere as invasive as duck weed is so you'll have no problems controlling it. It doesn't flower though, and I'm not sure of a floating plant that does and that would be suitable for your tank size. Hope this helps.
Nick