Duckweed or starwort?

In my quite recently (july) constructed pond a small plant has self seeded itself into the pond. Its leaves are close together and small and pointed at the ends, facing opposite to each other. There are usually three or four eaves per plant. The leaves are held above the water by thin black stalks that then travel down and root into the soil. I am not sure it is duckweed because it took several months to spread properly and is not quite as dense and vigorous as duckweed is normally considered to be. Could it be self seeded starwort?