Thanks for the comments. The background was pretty simple. I carved the basic shape out of styrofoam, concreted over the styrofoam, sealed the concrete with drylok concrete sealer, and siliconed it to the tank. The tank is a 40 gallon long. The Drylok comes in white and I used concrete die to color it. I painted on several shades of grey with the Drylok to get the color the way I liked it.