Backgrounds can add depth and interest to your aquarium as well as hide filters and other tank apparatus.
They can also server to block light from coming in (at least on one side) which can cut down on the growth of algae.
Backgrounds come in all different designs – you can get plain blue or black or underwater scenes. In my large saltwater tank I had a giant poster of a shark swimming towards you – it was the size of the tank and was a great conversation piece! If you have a reef tank you might want plain blue or black so you don’t detract from your reef, on the other hand you can really dress up a fresh water tank with a spiffy background.
You can also get molded resin backgrounds that are 3 dimensional and look like underwater roots or rock cliffs and attach to the inside of the tank.
Most backgrounds are plastic sheets that come on a roll and you cut off a piece that fits your tank. When selecting a background, keep in mind that it will look darker once you get it installed in a tank filled with water. Installation is easy – cut the background to the size of your tank then slip the top up under the lip of the tank cover and tape in place with water proof tape. Fit the background to the sides and tape to hold in place.