When you are adding the well water do you have the discharge raised above the water line so it splashes and helps add oxygen to it? If you have it entering the pond under the surface you are only adding water with no oxygen.
I even have gone so far as to have the well water discharge into a container of some sort so it bubbles around and then over flows or have a drain pipe and let that splash into the pond. This way it gets exposed to the air twice before entering the pond. I have this sytem on my little crayfish pond. I learned the hard way in the begining, added the well water directly via hose under the water line. All died with in no time of no oxygen.
