pond fish mass death

I have a fairly large landscape pond probably around 500 - 1,000 gallons. The gold fish have been in the pond for over two years. The water temperature has not changed. It was even colder, had ice this winter. It's cold probably around 40 degrees. I had about 25 - 30 gold fish. I haven't seen the littlier ones but all the big ones have died. Before they died they hang on the bottom of the pond. I had the water tested for ph and ammonia. Both were normal. I have been running the waterfall off the tap. I'm afraid to put more fish in the pond because I don't know what happened. There's a fair amount of debrie on the bottom. Any ideas?