Check all your readings again, that area at the bottom is a open wound, was it caused before the fish died or after? Check through the pond and look and see if you gained a occupant, We gained a small turtle during the storm, that's why i am suggesting this, If I had found a fish that looked like that in my pond and all water checks looked good before and after that would be one of the first things I would be concerned about and little turtle would be removed. However in my pond we get these little Musk turtles from time to time when we get hard rains and they seem to adapt well in the pond and have not caused serious problems so far. Although I have seen them get a fin of a fish during feeding time in the past. Also look for sharp edgings on rocks and deco in the pond. Is that the normal color of the fish? I have seen in the past where some blackening of the scales can be caused by ammonia fluctuations. Be sure and check your ammonia and nitrite closely. If anything went out of wack this would be where I would first look as it can cause issues almost exactly as you described, all look fine one day and a problem the next.
