It sounds to me like the tube is just freezing up and the water pump is working. Take the cover off and look at the tube to see if there is a low spot. That would be the point it's freezing at. If that's the case, use a wire tie to raise it up just a little and then check it again. If the impeller was to freeze up, you wouldn't be using it again because you would do damage to the impeller and or the shaft. The pump housing drains it's self out so I doubt it's freezing.