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Air pump only working with resistance?


Active Member
I needed to set up a small tank to separate a fish and grabbed a small top fin air pump I had sitting around. I plugged it in and it made air, but it wouldn't push air through a small sponge in water only 8" down. I made sure the tube wasn't blocked, etc. but all seemed okay. I figured it was just too old/weak and would need to be rebuilt.

But then, when I went to pull off the tube, the air started flowing out of the sponge. I stopped and it stopped flowing. I grabbed the airline to pull it off again and the air flowed. So I figured out that when I was squeezing the tube, the added resistance somehow made the air flow with enough pressure to drive the sponge filter. So I took off the hose, tied a knot in it loosely and then put the hose back on the pump. Up to the point of totally closing the tube, the tighter I make the knot, the higher the airflow... kind of counter to what I expected. So 2 questions:

1. Why? Not essential to know, but I'm curious.

2. Does this mean the pump is about dead and I better replace it asap or could I hope to get another month or so out of it for this temporary use?

It is just for one small wild betta, so if the pump stops it won't be an immediate problem, but I'd like to plan accordingly.


Active Member
I'm at a loss. that's weird.
Yes, totally opposite of what I'd expect, but I'm guessing it must be something about the diaphram wearing in such a way that it doesn't work without some back pressure on it. I'm going to have to put a valve inline since it slows down over time as the knot loosens.


Staff member
Very strange but also interesting. Maybe after it eventually fails it'd be worth taking apart to further investigate. I'm sure several of us would be curious, but the air pump manufacturer may also be interested to hear about the issue.

Frank Cowherd

Global Moderators
Staff member
Perhaps you have a crack in the housing or a leak somewhere in the airline. Your action in tying a knot might have closed the leak partially at least.

Wet Sleeves

CCA Members
what happens when you put your finger over the water hose? water goes shooting everywhere at a higher rate because you decreased the effective surface area where it comes out, which will increase back pressure. Your pump is dying and it doesnt have enough pressure to push through your airline or as Frank Cowherd Frank Cowherd said, it could be a crack or hole somewhere in the system.