Thanks again, Don! I think that finally answers all my questions... at least for now.

Oh, and about the Hydroton - I went to check out joshsfrogs.com, and when I clicked on "substrate" that was the first thing that popped up! So apparently there are a few folks out there using it for their frogs. They don't seem to be carrying it right now, though. But it looks like one bag ($20 on Amazon) is the perfect amount to fill an 18x18 area to a depth of just under 2 inches, so - perfect!