Egeria densa
Anacharis
Another old standby that is hardy and adaptable. This plant will also probably take on a different look in the aquarium than when purchased. Most offered for sale are very dark almost brown and quite thick. In the home it will generally become more green and depending on the light level may thin. In sunlight or good lighting this plant will be one of the first to start giving off little droplets of oxygen.
For more information about Anacharis please see my February 2006 column for TFH's "The Planted Tank," called
The Stemmed Plants.