With aquarium decorations, you must exercise great caution in selecting materials that will not damage aquarium water chemistry, which limits your choices for decor. With DIY decorations, you need to make sure no component will leach into the water and poison fish. However, you do have several options to build you own rock wall for the back of your aquarium.
DIY Live Rock
The most from scratch way to make a rock wall is to make your own fake rocks out of cement and crushed rock. This process creates an artificial version of live rock, a material composed of dead coral skeletons colonized by other organisms. You will need coral sand, concrete, crushed rocks and rock salt. Only select crushed rock from a pet shop to ensure it will not contain dangerous contaminants. First, create a bed of coral sand by spreading coral sand over a flat surface several inches deep. Shape the surface of the sand to create a mold for your concrete. Mix your crushed rock and your cement in a 1-to-3 ratio. At the last second, mix in rock salt. The amount of salt you add determines how porous the eventual rock will end up; more salt means more pores. Pour the mix over the sand, then allow it to set. Once its fully cured, soak your fake rock in fresh water, replacing it every few days until the soaking water stays at a specific gravity of 1.000. You will be left with a rock that resembles lava rock, with pores to encourage the growth of fish waste-eating bacteria.
PVC Rock Wall
You can also create a rock wall using pieces of existing rock or fake rocks with a PVC framework. Build a rough framework for your background with the pipe into any shape you want, including individual peaks and arches. However, a plain wall shape makes the most effective background. Once you finish the PVC framework, attach pieces of rock to it. For larger pieces, drill the rock with a 1/8-inch masonry bit, then attach them with nylon bolts. For smaller pieces, you can attach them directly with super glue and super glue accelerator. This method was originally intended for creating structures out of live rock, which requires working quickly to avoid losing valuable organisms. However, you can use this same technique with any aquarium-safe rock, real or synthetic.
You can also take a simpler route to create a rock background. In this method, all you have to do is photograph your favorite rocks. Then, take the photo to a local copy shop and have it blown up to the size of the back of your aquarium. Make sure you get the print on glossy paper. Then, tape the background to the back of the tank. This method adds an interesting aesthetic, requires less work, and has no chance of falling over and breaking your tank.
Take care to avoid adding anything to the aquarium that could damage water quality. For example, pay close attention to your adhesives. Make sure any glues or PVC cement has a "food safe" rating per the label. Also, do not forget to give adhesives enough time to cure; many products may produce toxic chemicals while curing. Additionally, always secure your rock-work so that it cannot tip and break the aquarium.