Just an update.
I serviced the filter today, along with my 50% water change.
A lot of debris in the bottom of the filter. This is a good thing, I want it to capture this stuff.
I noticed something when putting it back together, there isn't really a path for the water to go from the long sponges to the bio sponge at the bottom of the stack. There is a plastic wall that rises from the floor to keep everything stacked but no clear path to allow the water to enter the bio sponge and move up the column of baskets.
Can someone check this on their 405/406? I may have missed holes or something but I think this is why there is bypass, the water just goes where there's least resistance.
I didn't heat and bend the impeller cover back, I just cleaned it - I also cleaned the place it sits in the filter head. It was a little wobbly, but I wanna see how well this works...
2:00 minutes to fill a 5 gallon bucket before cleaning
1:50 seconds to fill a 5 gallon bucket after cleaning
Previously I had: 1:41 seconds - 178 GPH.
I hypothesize the impeller cover warping has something to do with this.
Well - If I can't DIY fix the issues, I'm going to put this on CL and sell it and replace it as soon as I can afford another brand that performs better.