Helanthium tenellum "Micro"
Helanthium tenellum (formally known as Echinodorus Tenellus before it was reclassified) is a carpeting grass-like plant that bears a close resemblance to a mix between dwarf hair grass and micro swords. This plant grows by sending out lateral runners just like hair grass and other swords and when grown under proper conditions can spread to cover the foreground of your aquarium. For smaller nano tanks, this plant can also serve as a short background plant but adequate lighting and CO2 are recommended.
Helanthium tenellum "Micro" is well known as "pygmy chain sword" in the hobby. While it is not difficult to cultivate in an aquarium, a true lawn effect can only happen with higher levels of light. It will also bring out a reddish hue in the leaves. Give this plant a rich substrate and it will take off fast. Helanthium tenellum aka Echinodorus tenellus is often used by Takashi Amano in his aquascapes.