Faking rocks

Fake rock and molded backgrounds. Maybe you'd expect me to be snide about fake rocks. After all, there are those plastic rocks that snap together like a Lego kit to form gravity-defying Rococo fantasy structures rivaling the ormolu rockwork of a French mantel clock, or the setting for a Road Runner cartoon. But frankly I've seen some astounding and great fake rocks cast in polyurethane resin and sponged with color, which are amazingly realistic. That's the material they're using in public aquaria. You can drill this inert material for plumbing or cut it to suit your needs. Filter inlets and outlets can be disguised. Heaters can be hidden too, as long as you've made plenty of allowance for sufficient water current to keep the confined area from overheating.
 
Have a look at the fake rockwork backgrounds cast in a polyurethane resin molded on real rock and roots, which the Swedish guys who invented and introduced in 1994  call "Back to Nature" Aquarium Backgrounds. Several kinds of full backgrounds, or modular rock sections you can piece together were briefly available in the U.S. but were too expensive I think to catch on here. There are various rock walls and Amazon root backdrops. 
An excellent step-by-step, fully illustrated description of a styrofoam "rock" background by Jay Luto was posted at www.aquabotanic.com 19 May 2003. Jay used pink styrofoam rather than white, with additional chunks siliconed to it, and West System two-part epoxy resin mixed with dye, in multiple coats, sprinkled with sand between coats.