What is it?

I have a plant in my aquarium I cannot identify. It came in as a "freeloader"
with some other plants. Maybe someone here can help identify it. The plant
has a similar growth habit to Riccia, it branches from the growth end, but
not as regularly. The plant is very thin, threadlike, with nodes. At the
nodes there are two small needle-like "leaves" and a small green "ball".  The
plant entwines other plants, at the surface, in the Riccia I have for ground
cover. I've tried picking it out, but it breaks easily and seems to be
pervasive. That's not really a problem, I would just like to know what it is.
Anyone know?
Neil Schneider
In sunny Poway, California where the temperature's reaching nearly 100F and
we don't have air conditioning.