I notice in your pic that the bottom foam on the intake side isn't quite fitted flat & squarely in the bottom corner. I try to flip my sponges at least once in a while if I can tell. Hmm, just going by memory the "thumbs" (barbs, nubs) are more toward the top but Dee could be right. I lay my grid flat, stack the media then lift up the sides to slide back in the filter box. You don't want to create "thumb grooves" in the sponge sides (eventually) by jamming it down all the time.
But it also could be the sponge ends are not getting rinsed quite as well as the middle so it bulges a bit where flow is better. I tend to wait until flow slows noticeably to clean, so it takes a long while to rinse & squeeze. I have a black sink so it's harder to tell if detritus is still rinsing out. But even with stainless steel I put a white plate (or dish or plastic lid) in the sink to see better when I "think" I'm done, there often more crap.
I've forgotten, do you clean both filters every week? Alternate is better, neither should need cleaning every week or even 2. Your floss looks kind of thick & not too even, but it's still fluffy & new in your pic...Is this still with just 2 goldfish?
