Things You'll Need
Razor knife or scissors
Spray bottle with water
Silicone aquarium caulking (optional)
If using a background on the inside of your tank, verify that it is designed to be immersed in water. Some might dissolve and leech harsh agents into your aquarium.
For interior backgrounds, all gaps around the edges need to be sealed completely to ensure water, rocks, debris and even fish can't get behind the backdrop.
The day before installing the background, roll it in the opposite direction and tape it in place. This eliminates some of the curling and helps flatten it out. Alternatively, if you have the space, lay the backdrop flat and weigh it down.
Follow the same process on the inside of your tank for rolled interior backdrops. Instead of using tape to seal it into place, use silicone aquarium caulking around the inside edges -- some may be self-adhesive. For a thicker three-dimensional background, use aquarium caulking along the edges to glue it in. Interior background adhesives need to dry at least 24 hours before setting up your tank.
Aquarium backgrounds give some oomph to your setup, making your tank look bigger and fuller, without taking up valuable space. You can add a backdrop on the outside of your tank at any time, whether the tank is full or empty. But if you prefer to place the image on the inside of the tank, it'll take some work. Interior backdrops require your tank to be completely empty, meaning you'll need to find temporary housing for your fish, if applicable. Either way, a new background gives a whole new life to your underwater setup.
Measure the dimensions of the tank with a measuring tape. Trim the background to match your measurements, by using a razor-sharp knife or scissors.
Clean the outside of the glass thoroughly. If your tank already houses fish, avoid spraying cleaning products onto the glass, so they don't splash into the water. Spray the cleaner onto a cloth and wipe the glass clean.
Move any obstructions, such as overhanging pumps, out of your way. Spray the outside of the surface with a light mist of water.
Align the backdrop with the top corners of your tank. If it's a larger tank, get a partner to hold one corner. Once it's straight, gently press just the top couple inches into place and set with several pieces of transparent tape along the top. For static-cling backdrops, peel away the backing before setting it in place. You do not need tape for static-cling backgrounds.
Spray some water on the outside of the backdrop. Starting in the center, hold the backdrop in place at the top and run your squeegee down the middle, then either straight downward or to either side, starting at that middle point. Work out the air bubbles.
Place transparent tape along the edges to secure the backdrop, if needed, once all of the air bubbles are pressed out. Exterior installation is complete at this point.