Aquarium fish produce lots of waste that eventually accumulates in the substrate. To keep the tank habitable, you must clean the gravel periodically. A common way to do so is with a siphon-style vacuum. Commercial gravel vacuums exist for every kind of tank, in many forms. If you're DIY-minded, you can easily build your own.
Tools and Materials for the Job
Gravel vacuums have two basic parts: a hose and a plastic funnel that attaches to one end of the hose. To make your own, you need a 2-liter bottle for the funnel and a hose long enough to reach from your aquarium to your bathtub or backyard -- do not drain the aquarium into a sink that your family uses. Most commercial gravel vacuums use clear tubes, but garden hoses work better for DIY vacuums. You will also need a tube of silicone, scissors and duct tape.
Building the Vacuum
Slice the bottom and top off the 2-liter bottle so you have an open-ended cylinder. Cut the cylinder down the middle to produce a flat plastic rectangle. Roll the cylinder into a funnel by wrapping one side of the plastic rectangle more tightly than the other -- the funnel's small opening should be about 1 inch in diameter, while the large opening should be between 2 and 3 inches in diameter. Slide the small opening of the funnel over the end of the hose -- adjust the size of the small opening so that it fits the hose tightly. Place the funnel all the way at the end of the hose. Tape the funnel into place on the hose. Apply a bead of silicone up the seam on the funnel; after it dries, apply tape liberally to ensure an air- and watertight seal.
Priming the Pump
Place the intake end of the hose -- the side with the funnel -- in the aquarium and secure it in place with a piece of tape. Be sure that the entire funnel is completely under water. Attach the other end of the hose to a water spigot and turn the water on slowly to fill the hose -- when no more air bubbles exit the hose, it is full of water. Turn off the water and crimp the end of the hose, which will prevent the water in the hose from draining into the aquarium. Take the outflow end of the hose and place it in a bathtub or out the window and into your back yard. Ensure that the tube has no kinks, and travels downhill from the aquarium over the entire length. When you are ready, release the crimp and the water will begin siphoning from the tank.
Tips for Use
Once the siphon is working, push the intake funnel into the gravel a bit to loosen the waste and debris. You can watch the vacuum suck out the dirty, dark colored water through the plastic funnel. Work your way around the tank slowly, tending to all areas of the gravel. If you want to give the tank a quick once over, do not press the funnel into the gravel -- merely skim the surface of the substrate slowly. For deeper cleanings, press the funnel into the gravel and move it around, but be aware that this will cause the water to become very cloudy for several hours until the dust filters out or settles. Once you have covered the entire footprint, remove the intake from the water, allow the hose to drain and store it for the next use.
Not a Drop to Drink
It is never necessary or advisable to place your mouth on the hose to instigate the siphon; as long as your hose is airtight and completely full of water, and the hose travels down from the aquarium -- it should spontaneously begin siphoning water from the tank as soon as you release the crimp.