Theres probally all sorts of good stuff in that pond sludge it makes sense, everything goes to the bottom when its dead. Like an underwater compost pile.That sludge on the bottom of a pond is greatly beneficial sir. I have a small pond that i do that every spring and my vegging plants love it. Very very potent stuff through. Smells horrible and is very messy . If you like fishing shouldn't be a problem. Also save all fish scraps and marine life in general. All that stuff is great , bones, shells, fins whatever. Eat it then recycle in your garden.