Hygrophila polysperma

Hygrophila polysperma from Southeast Asia is one of the hardiest aquarium plants available. The stems are about 25-40 cm long and 4-8 cm wide.
It's particularly good for beginners because it grows in almost any condition. They typically grow so quickly that it's important to prevent them from shading other plants. The leaves that remain on the surface form small new plants.
Hygrophila polysperma varies considerably in leaf shape and color, depending on the amount of light provided.