A "siphon" is a tube that transfers a liquid from one point to another. For aquarium purposes, it is a tube that transfers saltwater from your tank to a location below your tank, by utilizing gravity and atmospheric pressure. When both ends of a siphon are under equal atmospheric pressure, liquid will flow from the high end of the siphon to the lower portion because of gravity. In practical terms, this means if you fill a tube with the liquid you are transferring, and remove nearly all the air in the tube when you place one end lower than the other, it will suck water from the top and spit it out of the bottom. You can accomplish this through a variety of ways.
- Sucking the air out of the tube - notice I didn't say water. :) Place one end of the tube in the tank, hold it there, and suck on the other end the way you would a very large straw. When water rushes over the edge and gets near your mouth you have probably removed enough air form the tube and the water will begin flowing. Try not to get water in your mouth, if you do, spit it out immediately and clean it up afterwards, swallowing dirty seawater can make you sick, don't do it.
- Submerging the tube - This is especially useful for smaller siphons used in regular cleanings. 1/2" inch nylon tube found at a home improvement store is perfect for the task. To start a siphon with this method, completely submerge the tubing in your tank, making sure to work the air bubbles out of the tube if necessary. The tube should be 100% filled with water. Next, grab one end of the tube that is to remain in the tank and hold it in place under the water. Take the opposite end of the tubing and remove it from the tank with your thumb keeping the end in plugged and place it a bucket or other container, below the other end of the tube and outside of the tank. When you release your thumb water will begin flowing.
- Utilizing your return pump - Assuming you won't be removing enough water to make your sump go dry, you can use the return pump from your tank to start a siphon. Simply place one lower end outside of the tank in the container that is going to catch the drained water. Place the other end up to the return's exit to force water into the tube and to force the air out. Once the tube is filled with water, the end inside your tank can be removed from the return, and used to siphon water from anywhere in your tank and it will still flow down to the container below.
Things not to do when siphoning -
Never leave siphon unattended, the rushing water, changing water levels and other variables can move the tube from the position it should be in, and it can lead to spills.
Make sure your sump doesn't run dry if you haven't turned the pumps off first. Also, make sure that the water level in the tank does not go below the inlet for pumps and powerheads in your tank. If it will, you should turn those items off so they do not run dry.
To end/break a siphon -
Do not take the end of the tube that is in the tank out of the tank until you are done. If it comes above the water line air will enter the tube and break the siphon. When you are done siphoning, this is how you finish.
Taking the end of the siphon that was outside of the tank and bringing it up higher than the water line inside the tank will cause water to go back into the tank and air will fill the tube breaking the siphon.
Pro Tricks -
A siphon can be suspended by bringing the end outside of the tank a little bit higher than end inside of the tank, but lower than the water line. The rest of the tube should be left to hang in a U shaped Loop. When this happens water stops flowing because gravity and pressure are being equally applied. When you place the end outside of the tank lower again water will resume draining. This trick is useful when the container you are using has become full and you have to change where the water will be drained. It takes a little practice to do it without causing a spill.
If you have multiple tanks, a siphon that is started in one tank can be transferred to another if you are finished cleaning that one, (for example), without restarting the siphon. To do this, place your thumb over the end of the tube that is inside of the tank. Try to seal it with your thumb, the tighter the better. Water should stop flowing, but remain in the tube. (Because of pressure). Quickly move the end of the tube in the first tank to the second tank. Be as fast as possible as air is likely to seep back in if you wait to long and the seal will break. Once the transferred end of the tube is in the new tank, double check to make sure the drain end is still where it i supposed to be, and if it is release your thumb. Water will begin to drain from the new tank.