I use mineral oil. Spread it in a thin layer over the back of the tank ... I use a small brush for that. Then I lay the background over it, and squeegee the background to get all the airbubbles out of it, and to make sure all of the oil is evenly distributed. It's easier if the tank is laying on its side, but I've done it while upright as well. I've also taken cork tiles .... painted them with a metallic blue auto paint and siliconed them to the top and bottom rim (outside) of the tank.