I was actually going to post something about how I did it, but just havent got round to it yet.
I too have always found it tricky to attach the backgrounds to the glass and this is how I did one of my recent smaller tanks:
If using double sided tape, it just tends to make a mess and is difficult to
remove quickly and easily.
If using cellotape to secure the side panels, it tends to make the plastic
backing "wobble" and I can never get it taught enough. Also, the cellotape
looks horrible and if it gets wet, it tends to start peeling too.
Soooo..... I've decided to try something new and it look great ! Not only
that, but I can also remove it very easily to clean the glass and use the
magnetic glass cleaner etc.
Take some firm cardboard (exactly the right size that you need to use to
cover the glass, but no need to be thick cardboard) and cover it completely
with that clear sticky back plastic film (sorry, don't know what it called
properly, but it comes on a roll). This will prevent accidental water
splashes to cause the cardboard to swell up.
Then cut the backing paper so that all edges extend by a couple of inches
beyond the cardboard. Then wrap and stick as you would a gift.
You can then either just slide the backing in behind the tank if it's close to a wall. Any cables hanging off behind the tank will hold the backing close. Or if that doesn't quite work, just take a small piece of tack and put that in each corner.
Without backing
With backing