Sagittaria subulata (dwarf sag) bare root.

This 2-6" tall plant looks like underwater grass. Multiplies rapidly, and will populate any soil you plant it in. Has small white blossoms which will bloom out of the water if in shallow water. Very valuable bottom plant for absorbing excess nutrients from the water, competing with algae, and clarifying the water. Some gardeners use pea gravel on the pond bottom and plant hundreds of dwarf Sagittaria for a natural filter.