My original pond was close to 20 years old with a lot of muck before I started adding bacteria. The pond bottom was black and you would sink down about a foot if you tried to enter the pond. After 1 season of adding bacteria without aeration the pond bottom along the shore a couple of feet in went from black to looking almost like new fresh clay. The next season I added aeration to the formula and the cleared area went further in about a foot (but not a foot deeper).
I re-dug my pond 2 years ago to start fresh and have aerated and added bacteria since then, I have no muck at all (yet). I'll update as time passes. My pond is still young and small at 1/4 S.A., 1:3' slope, 13' deep and I aerate 24/7. If you don't aerate then the more efficient aerobic bacteria (vs. anaerobic bacteria) won't survive and the muck in the deeper areas won't get digested.