Your pond has sludge, which may be fine. A little bit of sludge is unavoidable but if your pond has you too much sludge it could become a big problem. The problem is that most of the sludge is often not penetrated into oxygen, they are very thick. The lack of oxygen inside it means that anaerobic bacteria are king, and their metabolism produces hydrogen sulfide. Hydrogen sulfide is the smell of rotten eggs, when you stir your bottom of the pond or clean your filter can smell. It's poisonous creatures in your pond, which means too much pond sludge can start to kill the aerobic bacteria and algae that are good for you; this means that more dead things are producing sludge (and less work to eat from Outside the sludge of the aerobic bacteria); this means more hydrogen sulfide to kill more things' even your plants and fish. Sludge also tends to form an oxygen seal layer along the bottom of your pond, which can kill the healthy algae at the bottom of your pond, causing more problems.
Sludge Treatment Finally, you have to add extra beneficial bacteria to keep your ponds back on track.