I had the same problem in my sump and yes I do use pre-filters... The pre-filter cannot prevent everything from getting thru. Inevitably some particles are gong to make it thru the pre filter and into the sump. I see nothing wrong with what you were attempting to do. I had a few "dead spots" in my sump where dentritus and other stuff would collect. I added a a powerful air pump that has multiple outlets and then attached 4 air disks two in each side of my sump in the dead spots to prevent stuff from piling up. I'm not sure what happens to all the crap but at least it's not collecting in the corners anymore.
I use 200 micron pre filters which I change every two days for clean ones.
I use 200 micron pre filters which I change every two days for clean ones.