thanks all. it's been tough finding the right rocks; and i need a backdrop or something; i keep planning on going to a fabric store (or something) and picking up some black felt for the backgrounds... that should be ok right?
in case you're wondering, i have eggcrate under the gravel/rocks. i also used eggcrate as light diffusers and to keep fish from jumping out. during the winter about 1/2" evaporates every day-- which is ok, as I keep it topped up, and I'm happy to have it providing the house with some humidity/heat.