Want to wrap up this issue by sharing what I have learnt here.. I think some of us using the Pro3 filters may face similar issue as what I have experienced and will benefit from my findings.
Essentially there is nothing wrong with the impeller.
What caused the problem of slow flow rate is the 1 component found on the motor cover of the filter. And the thing is, as layman, there is no way we will ever know the root cause because that wore out plastic will not cause external leaks, but instead will redirect water back into the filter. I will try to attach pic here showing you how the component has "worn out".
I'll be frank.. I think Eheim/QianHu owes their customer an update/announcement of this product improvement plan. This worn-out component obviously points to some sub-par engineering design for a German-brand filter.
It is ok not to give that replacement component for free. But I think it is fair to make this component widely available for sale to customers who owns Pro3. Afterall, it is only a pc of plastic.
QianHu suspected that component is the root cause. But that component is not a serviceable part as per Eheim Pro3 manual. And QianHu does not stock it. Somehow I think within Eheim, they probably has some sort of product improvement memo such that this same physical part is now available for filter after Pro3 model.
There seemed to be 2 types of this component online.. the one shown in the below weblink has a O-ring. The one shown on Eheim website does not come with the O-ring, and to-date, there is no part number for that O-ring.
You can get the part from the below website.
https://sg.miscota.com/fish/eheim/p-90430
Latest (online) manual of Pro3 filter shows this component
https://www.eheim-service.de/7211568-en
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