Robb D. VanPutte asks about riccia. 

It is a bryophyte, related to the mosses, but is a liverwort, which is a more
primitive genus.  Any single "tongue" or perhaps you might prefer the terms
"branch" or "antler" will bud and reproduce into a large colony when the
plant is happy. It normally floats, but will also attach itself to a log, etc

It grows like mad at the surface, about two to three inches below an ordinary
fluorescent bulb, forming dense, bright green mats. (Or, if unhappy, it just
turns yellow and dies.) It will NOT grow happily in a tank with any of the
duckweeds, as they "take over" the tank quite rapidly, crowding the Riccia
out.  It is also easily overtaken (translate - crowded out and totally
clogged up )by hair algae!

The conditions in the tank sound good, and you should be quite happy with
your Riccia. Personally, I would pull some small "starts" from your mat, and
float them in other tanks. My experience has been variable, the plant
flourishing next door to a tank in which it seemed to insist upon dying, with
the conditions apparently identical in the two tanks. Hence, my suggestion to
start it in several tanks.