Small Pond Turns Toxic Overnight

Yesterday morning I found a 5 inch goldfish floating in the small pond and the water looked very cloudy. I searched in the plants and found two more 5 inch goldfish that had died overnight. It was a surprise since we saw them swimming the night before. We decided to empty the pond and give the whole thing a good cleaning. We found two more large dead goldfish and a large tadpole and frog alive and well. We should have had 10 large goldfish in the small pond so the hawk or some other predator must be feeding in this pond. After cleaning and refilling, we let the pond water sit overnight. This afternoon we restocked the small pond with the 30 baby fish from the various buckets around the ponds.