Some years back I had a 5 acre pond and had the similar situation occur of a drought, low water, and much decayed weed and plants. We had a heavy, cool rain and the pond filled rapidly. The coolness caused the water to turn over and I lost all oxygen - killed almost all of my fish including catfish up to 16 lbs and bass up to 8 lbs. It was devastating. I was told the only thing that could have helped prevent this was an aerator - so, if your pond fills back up with all the decayed materials and you have a cool night, keep an eye out.