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I apologize that no one has responded to your question sooner. Different experts come online at various times. I just logged on and saw your question. My name is ***** ***** I’m a biologist with a special interest in fish. I'm sorry to hear of this problem.
When multiple deaths occur, there is usually something wrong with the water. Second most common is a viral disease. The first step is to test the water. when you do that, try to get one of the test kits that use test tubes and chemicals you add to water samples. The paper strips you dip in the water are inaccurate. When you get the results, please let me know what numbers you got in parts per million (ppm) for ammonia and nitrites.
Also, how many gallons is your pond? How long has it been set up?
Did you drain all of the water out?
Do the gills on the fish look red or inflamed?