Another name for this is purple waffle. While this plant is not fully aquatic it does work very well in a riparium setup. I picked up mine at the garden store, cleaned all the soil off the roots, and planted on a trellis raft.
As described above, this is a "terrarium plant". I bought a couple of pots of this at one of the box stores and planted in a suspended shower caddy under T5's and it does just great. The green top and purple undersides make for a neat splash of color on top of the tank.