Duckweed is a free-floating plant that can be seen on the surface, and is a bright, vibrant green. It is recognizable by its flat oval shape, varying slightly based on the variety, and is about 1/16 to 1/8 inches long with a singular root that dangles down below the surface. Duckweed reproduces by budding and fragmentation (one of the smallest plants known to do so!) and floats in large colonies. 
In aquariums, it works great to pull nitrates from your tank, especially in heavily populated tanks. But the plant itself has a habit of reproducing at a fast rate.
(Portions are measured without water, but will be shipped with water)