It is darn near impossible to keep a spot like that clean.
My pond has been full of water for a little over a year, and is getting ~9hrs aeration nightly.
I raked this pile of yech off of the gravel boat ramp yesterday.
It's a lovely mix of sago, FA, and tree leaves, with lots of tadpoles, baby crayfish, dragonfly nymphs, minnows, and other assorted underwater creatures.
Smells worse than a hog farmer's boot. There's a lot more stuff there than it seems. The heap is about knee-high, 3'x6'.