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Q & A: Fish Tank Siphon

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Most recent answer: 10/22/2007
Q:
My son got a fish tank. Not one of those dinky little bowls, mind you; a really big tank. Part of the maintenance for this mini-ocean is to siphon the um....er....stuff....out of the bottom every once in awhile. Siphon, like...plastic tube, inhale sharply, water comes out (ideally, not in your mouth; there’s that *stuff* ya know). So.....how does this siphon thing work?? I get how the water comes out when you inhale sharply, but why does it KEEP coming out? Is water like lemmings, blindly following the leader? Is it stuck together somehow? (But in that case, why is it such a bother to clean up when it spills?) Does it just LIKE going out of tubes? Enlighten me, oh wise ones! =)
- M. B. Kiefer (age way old)
Glenview, IL, USA
A:
Great question! I was wondering this exact same thing when cleaning a fish tank a while ago, and the answer is a lot simpler than it seems.

The device that you're talking about is basically just a long hose. (This is actually the same thing as if you use a hose to siphon gas out of a car.) You fill the hose up with water, then put one end in the fish tank and the other end in a bucket next to the fish tank. What happens is that when you hang the hose over the edge of the tank, the water starts to fall out of it in both directions. But the submerged end of the hose has water pressure pushing back on it, so the water on the other side falls out much more easily (into the bucket).

When the water falls down into the bucket, it lowers the pressure in the hose. Then the higher pressure water in the fish tank pushes into the hose, the same as the way water goes up in a straw when you lower the pressure on the top by sucking. You can think of it this way: if the tank water didn't follow along, you'd have a vacuum left in the hose- and you know the vacuum would suck water out of the tank. When the water leaves the hose, it's lower than the water level in the tank, so the net effect is just to have water falling.

As for your other comment, you don't actually have to use your mouth to get this started. All you have to do is get the hose completely filled up with water. You can do this by submerging the entire hose in the fish tank to fill it. Then put your fingers over the ends and move one end into the bucket.

On the other question you raise- does water stick together- the answer is yes, some. Just like you say, if it stuck together much more it wouldn't break up into lots of little drops when you spill some. But it doesn't just spread out all over the place as a smooth layer. It beads up, because it does stick to itself some. That's the same effect (surface tension) that makes it possible to slightly overfill a glass of water. I'm not sure, but it seems like that helps keep the flow in your siphon from breaking into counterflows of air and water, which you could imagine happening in a big tube, and which would keep the siphon from working.

-Tamara

(published on 10/22/2007)

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