What Is a Siphon and How Does a Siphon Work?
(The First Tank Guide)
The term "siphon" is both a noun referring to a tube or pipe which allows water (or another liquid) to flow from a high place, over a higher point and then down to a lower level and a verb meaning the action of water (or another liquid) flowing from a high place through a tube or pipe over some higher point and then down to a lower level.
How Does a Siphon Work?
A siphon works by gravity. Yes, gravity can be used to push water up over a hill or over the edge of your fish tank.
Here's how it works. Gravity is pulling down on all the water in the fish tank. If there were a drain in the bottom of the fish tank and you opened the drain, the water would all come out, right?
The siphon sort of opens that drain.
The siphon only works if there is water in the siphon tube. If you stick one end of your siphon tube in the fish tank and drop the other on the floor, nothing happens, right?
So, to start your siphon you're really just getting water in to the siphon tube and up over that top point.
Once you get the water started over the highest point, that water can then just flow down onto the floor (or hopefully into your bucket if you were paying attention).
But that only works for the little bit that came over the top, right?
Well, sort of. Remember I mentioned gravity up above, and pointed out that gravity is pulling all the water in the fish tank down? Well, gravity is also pulling down on the water in the siphon tube. The water that made it over the high point in the siphon wants to just flow down the hill. The fact that this is flowing down contributes to pulling more water over the high point, because, well, otherwise there'd be a vacuum, and nature abhors a vacuum.
In this way, water will continue to flow, even though it has to go up hill a little, as long as the bucket end of the siphon is lower than the surface of the water in the fish tank, the tube is unobstructed, and the end of the siphon in the fish tank is below the water level in the fish tank.
Stopping a Siphon
There are two different ways of "stopping" a siphon. You can stop the siphon by stopping the flow of water, or you can break the siphon by stopping the flow of water in a way that the flow cannot easily resume.
There are several ways to stop a siphon. for an aquarium siphon, the most common way of stopping a siphon is to put your finger over the "out" end of the tube, and just hold the water from flowing. This can also be accomplished with a valve.
A siphon will also stop if the water level is equalized. That is to say, if the water level at the "out" end of the siphon reaches as high as the water level at the "in" end of the siphon, the flow of water will stop.
Generally there are two ways to break a siphon. If either end of the siphon tube is raised higher than the surface level of the water you are draining, the siphon will be broken. A siphon can also be broken (though I would not recommend it for an aquarium siphon you have to use again next week) buy puncturing the line at or near the top of the line and allowing air into the tube.