I built this 18"^3 vivarium for two highland r. variabilis. I put this together about a month ago with a clay background of redart, peat, bentonite and calcium carbonate. The fan really improves the visibility through the glass.
The frogs are still in a ten gallon tank until they are just a little bigger. There is a smaller fan in the ten gallon tank, and I think it has made a big improvement in the behavior of the frogs. They don't seem to hide as much now.