This morning I heard my Fluval 405 making some odd noise. I check it out and it is no longer working although the motor sounds like it is running. I try taking it apart and making sure there are no obstructions or anything obvious, put it back together and it starts working although a very weak output.....then stops again. I pick up a new impeller and try that. It is working for now but the old impeller seemed fine. What goes wrong with impellers and why do they need replaced?