Creating your own background is a whole diffrent story, you should do research on how to do it. It is cool when you do it and makes your cage uniqe and there safe. there is a whole lot of work that goes into them though. The oil slick she is talking about is something that gets on the surface of the water, kinda like if oil is on the surface of the water. It is harmless to the fish and i just let it be as it goes away in a water change or 2. If you soak it for a week or 2 before putting it into your tank then you are ok.