Some pet fish enjoy having hiding spots they can rest in away from other fish or excessive light. Making a small tunnel in an aquarium will help these fish feel more at home and increase the realism of a fish tank. Use a variety of basic materials to craft a tunnel that will be enjoyed by fish of many types.
Cut a length of PVC pipe to the size you want the tunnel in your aquarium to be. The pipe should be 3 to 4 inches in diameter, but depending on the fish that it is intended for and how much room you have it can be smaller or larger. You can make it long enough for a fish to completely hide in or simply make it a fun swim-through decoration in your fish tank.
Use a chemical-free scrub pad or a file to smooth all the inner and outer edges of the piece of PVC pipe. This will prevent scale and skin damage to the fish in your aquarium. Clean the piece of pipe thoroughly with clean water before use.
Spread a layer of aquarium-grade silicone sealant along the length of the PVC pipe tunnel. Press small rocks, pebbles or aquarium gravel that match your existing fish tank substrate into the sealant. Be sure the rocks and gravel are securely attached to the silicone so that they will not fall off over time or when your pet fish rub up against them.
Stick on more small rocks and gravel with silicone sealant until the entire piece of PVC pipe is covered. You can also sprinkle sand or other small substrate between the pieces of gravel to cover the pipe completely. Avoid having any sealant showing through the rock layer.
Place the tunnel in your aquarium where you think the fish will enjoy it. Be sure there is enough room on either end for the fish to enter and exit without having to scrape their bodies on the edges of the tunnel or other aquarium decorations.