Cabomba caroliniana
This is a nice plant, but not the easiest to grow. It needs a lot of strong light. Some Cabomba have a lot of red in them. Cabomba also tends to send down a lot of roots, which can be less attractive than might be desired in some tanks. I find that in natural sunlight, through windows the plant is greener, and sends down less of the roots along the stem.
For more information about Cabomba please see my February 2006 column for TFH's "The Planted Tank," called The Stemmed Plants.