yoli i have read a few books on pond building this summer but have gained most of my information on this site. i have read where some people have used a dredge to clean out and deepen there ponds. we opted not to go that route but maybe some of the pond experts on this site can tell of there experinces with dregeing. i was concerned with losing the seal and not having the pond fill back up or leak. you could deepen it with just a back hoe if they can get close enough. we rented a 8 hp gas pump to lower are pond so the equipment could clean up the edges, it did not take that long to draw it down. i am hopeing that airation will start eating away at some of the muck and over time it will be considerably less. i have seen claims of ponds using both airation and bacteria say you can lose as much as 5" of muck per year. my guess is more like 1-2" sounds reasonable. hope this is of some help to you this is my first experience with ponds so i am learning as i go as well.