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Wood is a natural decoration that adds to your aquarium's look. Algae that grows on the surface of wood can provide food for certain kinds of fish. Driftwood is a popular choice because of its twisting shape. You can sew carpet moss onto a piece of driftwood to combine wood and plant life, making the tank more resemble a natural underwater habitat. Boil any structure you plan to put in the tank in water before introducing it to your tank. Boiling will sterilize your wood or rock to make it safe for your aquarium. Boiling wood will kill any bacteria on it and remove tannins that can stain the water. Fish will also often hide behind rocks. Rocks have a variety of shapes and colors, so you can look for rocks that you think add to your overall theme. Choose rocks that speak to your sense of style.  Avoid having large rocks directly touch the glass. Over time, the rocks may damage the glass, causing stress fractures or cracks.  After boiling rocks, soak them overnight and then brush them to remove loose particles. These can include the typical castles and pirate treasure, or you can go for something more imaginative. Consider getting a few decorations in different sizes to provide different territories. Like plants, structures give fish hiding places that they enjoy. Some structures can add oxygen to the water. Be very careful when buying decorations with holes or small spaces. While these can be ideal hiding places for smaller fish, larger fish can become stuck and get injured or even die trying to get out. Be sure any hole or archway is significantly larger than your largest fish. If you have young fish, keep in mind their full grown size when picking out such decorations. Experiment with the placement of your decorations to find something that is aesthetically pleasing to you. Having one main decoration that is larger than everything else can provide a good focal point, but also make sure it won't crowd your tank. Balance items with empty space. If one side of your aquarium is going to face a wall, consider attaching a background to it. These simple sheets of plastic can be bought at any pet store, and come printed with beautiful pictures, from undersea plants to deserts. A simple black background will lend a very professional look, hiding cords and tubing and making the colors in your aquarium stand out, especially if you have a lot of plants. If your aquarium is looking a little sparse, a background printed with plants or coral reefs can give the impression of a much lusher environment.  Choose a background that fits your theme. An image of a coral reef suggests a natural space. You could go in the other direction with something like a space-themed background if you want a more unique theme. Measure your tank before going to the store, and buy a piece at least 1" longer than your tank to account for uneven edges. When you get home, cut it down to the proper size and tape it to the outside of the back of your aquarium.

Summary:
Decorate your tank with wood to provide hiding space and a potential food source. Decorate your tank with rocks to make the landscape at the bottom of the tank more interesting. Add some artificial decorative elements for a more fantastical look. Arrange the items in the tank artfully without overcrowding the space. Purchase a background to enhance the overall aesthetic appeal of the aquarium display.