Riccia isn't nearly as good as a plant that actually breaks the surface, because plants absorb CO2 from the air much faster than from water. Duckweed is also nearly impossible to kill, and will grow extremely fast in a tank with no CO2, moderate lighting and no ferts other than fishfood.
Giant Duckweed is similar to duckweed, but more managable. Giant Salvinia is like duckweed, but the leaves are 1 inch across and there's 3-5 leaves to a plant.