Hi,
Back when we were living in Seattle, we had a huge batch of water letuce growing covering our 55 gallon. Mind you, we started with only about a handful lol. They looked great, and the roots reached down to almost 1/3 of the tank's height. It was a lot of fun watching our fish navigate the maze of roots. They are a lot less messy than duckweed. The only problem is that they blocked the light from reaching our other plants.
I don't know how heavily planted your tank is, but if it is heavily planted you might consider growing some of them to the top of the water instead of using floating plants. I just ordered some corkscrew vals and will probably let them drape across the water, and my swords are providing the gouramis a lot of shade/hidding space already.
Good luck!
Hyla