Rotala rotundifolia
An attractive easy to grow stem plant. Will grow in moderate to high level lighting. It does take on a pinkish tint in brighter light, and will also spread across the gravel. This plant grows so fast for my under the shoplights that I generally just chop off the whole stand to an inch or two from the gravel base and let them grow out again.
For more information about Rotala please see my February 2006 column for TFH's "The Planted Tank," called The Stemmed Plants.