I am having trouble attaching my store bought aquarium background to my tank
. How do you do it? Is the background supposed to go inside the tank or something?

I got it on, the vaseline did the trick (but I ended up using water and cooking oil too). There are still some air bubbles but I did the best I could to get them out (I used a rigid piece of plasic). All in all I am happy with the way it came out.