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.