Need suggestions for floating plants
 

I need suggestions for floating plants to put in my paludarium.
The paludarium is 4' x 8' x 6' tall. The lighting is 3' or so above the surface of the water, and appears to be sufficient for the terrestrial plants. However, I have tried water hyacinth and water lettuce, and I can't keep them alive, presumably due to the distance from the light.
I have a large water surface area that I would like to cover with floating plants.
Any ideas?

Duckweed is almost impossible to get rid of once it gets started, Salvinia is a larger-leafed but similar plant that grows in chains rather than masses of single plants like duckweed.