I have one of these old air-driven aquarium filters, but I’m wondering a bit about how the tapering of the intake and exhaust tubes effects the deal.

So at first thought it seems very simple, the air comes down from a pump through a line into the tube on the right, as the diameter of the tube increases the speed of the air should decrease. Then that air spills/bubbles into the tube on the left (which is open on the bottom allowing water in), and as it rises in the tube its speed should increase as the diameter of the tube narrows.

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But then I began thinking it about it (dangerous), air bubbles travel faster the larger they get, and unenclosed by a tube a bubble will expand as the pressure of the water decreases in correlation to the decrease in the depth of the water. But these bubbles are in a tube which gets more constrictive as the bubble moves up. So should the bubble exit the tube at speed faster than it entered? Or will the tube slow it down? Or does it make any difference at all?