I clean the whole filter case but concentrate on getting the well the impeller drive sits in absolutely spotless. I also remove the impeller and wash it very well separately. Since the clearance between the impeller magnet and that well is tight, any speck of dirt in there means the impeller will not turn. Be careful with the impeller, those little blades that spin around are fragile and are all you have to move water through the filter. The rubbery bits on each end of the impeller shaft are absolutely necessary, do not lose them. They act as bearings for the impeller shaft.