In the “good old days” most aquaria were constructed differently than they are today. To build a watertight glass sided container it was necessary to have strips of material, usually metal, at the edges and the corners. The glass was held in place with glue or caulk. Sometimes as the tank aged the caulk would turn dark, crack and ooze out onto the glass. The sealant would eventually dry out and leak. Fortunately for the fish, the leaks would be a slow process and an aquarist would have warnings before it was necessary to replace the tank.
Today fish tanks are built with strong epoxy glues eliminating the need for the corner strips. The unencumbered look is well liked by most aquarium owners but it does present the aquarist with a problem. In the past the unsightly tangle of tubes and cords that are associated with a working aquarium could be partially hidden by the side strips. In order to add interest and help hide unsightly tubing an aquarium background is often used.
Hiding tubes and cords is not the only reason to use aquarium backgrounds. Background scenes can add color or set a theme for the tank. The right background can add depth to an aquarium. A mood or theme can be established by using the appropriate background.
Usually a background comes as a sheet of printed-paper the size to fit your aquarium. The sheet is then attached to the tank either by a peel and stick or by an edge adhesive. There are also contoured sheets of plastic that can be fitted to the inside of the tank. The contoured sheets provide interest and can help eliminate the need for other aquarium ornaments. Occasionally a skilled craftsman will make an insert that can be placed in the tank. If you choose to build a background for your aquarium, keep in mind that the materials you use should be safe for aquarium use (see a post about the importance of basic water quality).
Buying a background sheet can be relatively inexpensive. They can be ordered from fish supply houses or picked up at a local pet store. If so desired, moods and themes can be changed often. Be imaginative. Why not let a child draw a picture and glue it to the aquarium? Another way to personalize the scene could be done by clipping pictures of family members and gluing them onto the background. Imagine the fun of having Uncle Fred waving to you from behind a bolder. The possibilities are endless.