Tank Wallpaper

Do you mean those sheets of plastic with either a scene printed on them, or plain black or blue?

The easiest way is to sellotape them on. If, like my tanks, yours has a plastic trim at the top and/or bottom, you can slide the top/bottom of the plastic sheet between the glass and the trim, which saves having to sellotape that edge.
On my 125 litre, there's a trim top and bottom, so those edges are slid in the gap. Where I used to have it, that was enough but I moved it, and people walking past caused a draught and the background kept falling off, so its now got sellotape down the exposed edge.
My 50 litre has a trim on the bottom only, so the background is slid into that and sellotaped at the top and sides.

Some people use water or cooking oil - the idea there is you spray them on the tank then using something like a credit card, smooth the background over the glass using the card to push the air bubbles out. I could never get that to work long term. After a couple of weeks, air bubbles would make their way between the plastic and glass and look awful.

Sunflower oil (quite a lot) has worked a treat for me and lasted over a year with no sign of bubbling. I had to pour the oil on the backing while it was flat on a work-top, spread it around with kitchen towel so it was a good wet layer, then roll the backing into place on the tank. Skim the bubbles out with a credit card or similar and voilą.

Thankyou everybody for your kind advise, it has all been very constructive.

Bas.