Aerators are more about water movement than aeration. With the idea being that they mix the water layers so the deepest water is brought to the surface. You should have started while the water was cold and a thermocline had not yet set up, or installed the airstone in shallow water and slowly moved deeper( 1 or 2 feet a day) to prevent the anoxic water from mixing too quickly. You likely released an incredible amount of nutrients as well with it being an old pond.
