Glad I saw this. My 306 is having similar issues. On my 75 I have the 306 and a 305. The 305 (and my 405 in another tank have very strong flow) but the 306 is weak like in your video. I cleaned everything aka took my garden hose to it, and the flow did not improve at all. So a few days later I took out all my floss and those polishing pads out. The only media in it is the ceramic rings and those tall pre filter pads......and still a pitiful flow. So should I take the pre filter pads out?
Edit: Oh the 306 is only 6 months old. While the 305 I have had up and running for probably 6 years.
On the 305, is the design of the impeller cover different to the 306? As I understand it ( I may be wrong ), the design of the impeller "well" is different.