Forgive me for saying so fella, but that cover does not look all that flat to me. Does it rock from side to side when you press down on the sides away from the hinge and clip?
PS. Given up fiddling with mine. Flow had gone low again today, when I checked the impeller cover was out of shape again. I think this is a design fault, the cover being secured at only two points. In all other filters that I have had in the past that cover is held VERY firmly in place. I brought this filter because of the amount of time I was spending servicing my two ancient Sera 900s. I'm spending more and more time faffing around with the 406. Nufs e nuf. I'm going for a Ehiem Pro 600.
Well, Sir, I stand corrected
The cover was warped - it rocked from side to side when I pressed down. I took it out, boiled some water, dipped and bent it in the reverse way. I replaced and tested the flow with a 5-gallon bucket.
1:41 seconds - 178 GPH, better than before (159 GPH) even without media (173 GPH), LOL
This is a design flaw or a quality control flaw if it's happening with other people's filters. I service my filter every 2 months so there should be no reason for it to run "hot" enough to warp this piece.
Finally, continuous heating and reshaping will weakening the plastic and cause failure - which catastrophic if it's running and the impeller gets impacted by a piece.
I'm thinking Fluval may intend this to be a part that needs to be replaced every couple of months at $15? http://www.amazon.com/Fluval-306-406...impeller+cover
Now back to my hypothesis of bypass - I need time to see if there is detritus settling on the plants over the next few weeks.