After I do a water change, my Aquaclear 50 barely works. Before I plug the filter back in after the water change, I fill it up with water and then plug it in, but then it only trickles water out. I don't understand why it does this. If I take the whole filter apart, rinse everything off, put vaseline on the impeller shaft, and put everything back together, then it flows water out properly. There is also a bubble on the intake shaft (on the bend where it goes down with the piece of plastic on top to control flow), not sure if this matters though. Does anyone know what this happens? The impeller is sucking up water obviously, but it feels like it isn't enough for a steady flow.