looks good mitch, i made my background a completely different way so its cool to see different techniques and ways to make them

excellent. what kind of concrete is it, and how do you apply it?
And why keep the background off the bottom when you siliconed it in?

Curtis, thanks!

Mark, thanks as well. the background being off the bottom of the tank was not intended. Problem I had is the tank was up and running while I was making the background so I couldn't get exact measurements. As for concrete, I started with standard quikrete and it didn't work out as well as I had hoped. Then doing so research online someone mentioned using quikrete's driveway resurfacer... Took all of the work out of it, wish I had started with that. Color was added by using concrete dye.

Right. I just thought there was a Stanley knife or something holding it of the bottom in the pic with the bricks.It looks really good, especially with the holes/caves.