just to share my experience, there is a white plastic stick and two small rubber cover goes through the whole impeller. The set up MUST be as followed:
Rubber Cover --> Plastic Stick --> Impeller --> Impeller Cover --> Rubber Cover.
Last time I made a mistake as
Rubber Cover --> Plastic Stick --> Impeller --> Rubber Cover --> Impeller Cover and I had the same issue as yours.
Anyway, not to blame Eheim but I feel that Professional II series are quite troublesome canisters. If giving a choice, I will still stick with the round-shape classic models.
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