Jeremy
I doubt U. gibba cares what is on the bottom of the aquarium as long as
it doesn't make the water alkaline. In the wild I've seen U. gibba
growing in a lake lying over the top of nearly molten bitumen (the stuff
they used to put on roads). You could use stone chippings, standard
aquarium gravel, glass. If you want to use sphagnum moss peat, you'll
need to cover it with stones of some sort or it will mess up the tank.
regards
Paul