The plant is Egeria densa, formerley known as Elodea densa/canadensis and commonly called Ancharis. Easy to grow plant. A real cosmopolitan species that occurs over much of the temperate world. In low light it gets spindly, and may break off and float free. In good light it has tight leaves and grows very fast. A good plant for beginners as it has an allelopic reaction to some algae types, (produces chemicals which inhibt algae).