I have a big bunch of that in my 15gallon, it started off a few months ago, bought from someone on another forum. Came as 5 sprigs, each 5cm long, and is now maybe 20-25cm in diameter and about 10cm high in the middle of the bunch: I think I've had it in the tank for 3 months now. Grows pretty well, my tank is low light (1wpg) and minimal CO2. I'll be putting some into my aquacube soon which is really why I bought it. I just tied the sprig to a pebble and let it get on with itself!
It does send down very fine thread-like 'roots' and it seems to anchor to things 'to a degree' though unsure if it is actually rooting.