Above is a picture of the background from my tank. I love the realistic texture of the rocks and the fact that they stick out (some pretty far) into the tank. There are also caves built into the background that smaller fish can swim through (covered in pic by my giant Oscar).
This background was made with 2″ sheets of white styrafoam insulation, silicone and cement. First I carved up the styrafoam into rocky shapes and bonded together with silicone. I made it in sections so I could put them in the tank after. Where there are caves I put 2 layers of styrafoam hollowing out the back layer.
Once the styrafoam was sculpted I started to paint/pour cement right on top. I did 3 layers and I added black and white cement coloring to accent dark and light spots. I tried to keep it wet with a spray bottle while I was making it so the cement will cure properly.
Then the panels went into the tank. They were siliconed to the back and I rinsed 3 or 4 times over a week period until my ph levels dropped to a safe level.
It took around 2 weeks to make the background in my spare time and 1 week to rinse but looks awesome, well worth it. There are a couple spots where the cement has eroded off, I plan on covering these with Java moss.