Create a stable rock wall, using Live Rock, dead rock, or a combination, by placing a framework of PVC piping under the substrate, and then placing substrate (sand, or gravel) around the PVC, but have the rock wall rest on the PVC so that tunnelling pet fish can still tunnel without undermining the rock wall support. Be careful not to use PVC glue. Use aquarium sealant, or other glues designed specifically for fish tanks. Another option is using a hammer to drive the PVC piping and PVC “T's" and other connectors together. Or assemble the PVC piping and connectors then drill through them both and apply plastic strips as though you would a plastic nail to lock the connectors and pipe together. Do not use metals, such as wire, or screws as they are very toxic to your aquarium fish. Distribute the rock wall weight as evenly as possible along the length of your fish tank.
Assemble, hammer and/or drill the PVC support structure outside of the fish tank, then reassemble it inside the tank after all the drilling, cutting and sizing, is completed. You can disassemble a large rock wall support system into two or more smaller pieces that fit into your tank easier. Do not drill the PVC while it is under water (sorry, couldn't resist ;-).
Position the rock wall away from the sides of the fish tank so that you have sufficient room to clean off the algae that will eventually grow there. Leave sufficient space to permit vacuuming up the debris that accumulates on the bottom. TIP: experts aim some water circulation to push debris into specific fish tank locations designed to be easy to access for maintenance.
Depending upon the size of your aquarium and the needs of your pet fish species, you could setup your rock wall into various shapes that balance out your aquascape visually while considering weight distribution. Arranging your Live Rock wall into a “W" when viewed from above would be ideal, but would require more space than if you setup your rock wall into a “V" when viewed from above. If you have sufficient space and have more free swimming pet fish than tunnelling fish, or cave dwelling fish, you could setup your rock wall into two “L's" (when viewed from above), one on the left side, one on the right side, of your aquarium, leaving free space in between. This is an ideal way to “bracket" your aquascape and accentuate the aquarium environment. If you have a 90 gallon fish tank, or larger, you may be able to incorporate a combination of two or more of these shapes. For instance “L's" on both sides, with a “V" or a “W" in the middle.
Be sure to leave holes extending front to back throughout your rock wall, especially if you will be using Live Rock, or Live Corals, to eliminate any dead spots in water circulation. These aquarium inhabitants depend on water circulation to provide food, and sweep away waste.
You should consider the habitats of the types of pet fish species that you will be keeping in your fish tank before beginning the aquarium design. These suggestions apply for saltwater fish tanks, or freshwater fish tanks alike, regardless of whether you have 20 gallon fish tanks, or 200 gallon fish tanks.
Dan Galen is an author and enthusiast on fish tank decorations and all things aquatic and the owner of http://www.FishTankShop.com To learn more about aquascaping and setting up your fish tanks environment please visit the site.