I just got done doing a water change and doing a bottom vac, and I noticed that the worst spot in the tank for dirt and other fishy waste is right under the filter intake. It seems kind of ironic that the dirtiest place is right under the filter intake so I was wondering if there is a reason for this?

The suction is strong enough to pull it over but not pull it up. Sounds like under/around my sponge filters. Yick! Also under my bubble wallls there are sometimes little piles of... well, you know!