American Elodea:

American Elodea is native to North America where it is an important plant for aquatic ecosystems.  It is also used as aquarium vegetation.  Elodea is entirely submerged except for the small white flowers that break the surface.  It can be identified by having 3 leaves per whirl on the stems.  Elodea does not heavily rely on seed production to reproduce.  In the fall, stalks detach from the parent plant, float away, root, and form a new plant.  This reproduction behavior allows these plants to form dense mats that are beneficial for fish habitats but deter recreational activities such as boating and swimming.  Elodea can be treated with Reward/Hydrothol.