Best way to do it .... although it's going to be a bit more difficult since the tank is setup . . . but you can still do it. First pull out all of the heaters, filters, hoods, etc.
Remove all of the old stuff glue, tape, etc., and clean the glass really well with alcohol. I assume your background is some kind of plastic or foil. Clean that with alcohol as well. This method won't work with paper. Trim the background to fit the glass from edge to edge and top to bottom just above the frame. You only want the background to fit the glass. Get some light oil . . . 3-in-1, or mineral oil from the drugstore.
Spread a thin layer of oil all over the glass . . . from edge to edge . . . top to bottom . . . make sure it covers the entire glass . . . lay the background on the oil and working from the center to the edge with a squeegee or credit card . . . the excess oil should ooze out the edges and bottom . . . just wipe off the excess. It will last for years and you won't have to screw around with taping and retaping, plus it looks really neat.
