Ground cover

What do you use for ground (technically gravel) cover ?

I have been using riccia fluitans recently as ground cover. I used the 
the method described in Amano's Nature World Aquarium by tying them to
rocks to weigh them down. I tied them with the smallest diameter "invisible" 
fishing line. This worked out well. It is really invisible even with close 
inspection and it is durable (still no problem after two months or so).

It looks like you have a lawn in your tank. And the best part of it, when you
trim the lawn, the trimmings will float and grow more riccia on the surface.
So you can start with a small amount of riccia and end up with a whole lawn
and more. The color is very pretty bright green (under 2 watt per gallon
amount of light).

The draw back of having floating riccia is that it's hard to clean the
water surface from floating dead leaves without sucking a significant amount
of the riccia itself.

I also want to try Glossostigma elatinoides but still could not find them
here in the US. Any tips or hints ? Erik (Olson), did you find any ?