Sometimes a store-bought aquarium background just doesn't cut it when you want something special. You can customise your own background with a bit of work by creating a faux stone effect, and the end result looks very natural. While you're at it, you can add some natural wood. The tannins in wood help to keep the water slightly acidic and stable. When choosing a wood, make sure that it is not fresh green wood, but old, dry wood, or it might rot quickly. Brushwood (common branches) is fine so long as it is dried, as are store-bought driftwood and bogwood.
Bring a large pot of water to a boil on a stove. Add the wood and let it soak for a good ten minutes. Boiling wood leeches out some of the excess tannins and allows it to sink in an aquarium. Boiling the wood also takes care of a lot of bad bacteria that you might otherwise introduce to your tank from the outside world. Next allow the wood to fully dry.
Don gloves. Empty your tank if it is not empty already, keeping the fish in an alternate aquarium. This project is better suited for a new aquarium that you are setting up rather than an already established aquarium.
Set a thin block of styrofoam at the back of your aquarium, cut to your tank's dimensions.
Break off ragged pieces of thin styrofoam and glue them all over the styrofoam backing with a tube of aquarium silicone. Aquarium silicone is primarily used for aquarium seams and hardens into rubbery translucent glue. You can arrange the styrofoam however you like. Just try to make the pieces look like layers of rock. You can even form the styrofoam into caves if you like. Embed your wood into the styrofoam by drilling it in, using a knife if necessary.
Go around the edge of your styrofoam backdrop with more silicone. Seal off any edges where the tank and the styrofoam come into contact. This will keep water from flooding behind your backdrop.
Add more styrofoam to the backdrop on the lower part to create a three-dimensional stepped effect that resembles stone.
Mix up a bucket of Quikrete, adding four parts water to one part conctrete. Mix it together for two minutes. Use a large, old paintbrush to paint the concerete over your styrofoam backdrop. Work quickly, as the Quikrete has a working time of ten minutes to half an hour. However, do not leave any spot bare. Work it around any wood you embedded into the styrofoam. Let the concrete harden for a full day.
Mix up another batch of Quikrete and paint a second layer onto your backdrop. You can add colouring as per the Quikrete instructions if you have any. Otherwise, the Quikrete will harden into a white surface. Be sure to work the concrete over the areas where the backdrop joins with the tank.
Let the concrete set for three days before adding any water or fish.