I'll try to take more closeup pictures of the background while I'm finishing the actual tank, in the next week or so.

Basic procedure:
Materials - 2" styrofoam sheets, silicone, pre-mixed tile grout, gray primer, flathead screwdriver, gloves, knife

I cut the styrofoam to fit the back of the tank. The back wall is composed of 2 pieces, with the cut to fit them together curved to make them blend better. The larger back rocks were cut out of these initial sheets. The crevices were made with the screwdriver, a putty knife can be used to rough up the styrofoam.
Also, smaller rocks were cut out of extra styrofoam and siliconed in place, allowing these to dry for 24 hours.

Then, the grout is spread over the styrofoam, let dry for 24 hours, then put a second layer on. This smooths out the bumps from the rough styrofoam a bit, to make it look more like typical rock indentations. I also added extra grout around the siliconed rock pieces, in order to make a smooth transition from the overall background piece to each piece of rock, so it looked like it was all 1 piece. Let the second grout coat dry for 48 hours to be sure it's ready for painting (if necessary).

I had white grout, which I spraypainted with gray primer. I like the way the paint came out uneven and some of the original grout color shows through in a couple of spots - i think it makes it look more like real rock, which does not have a uniform color. The whole thing is spraypainted and let to dry and air out for 24-48 hours.

Jim