Do you remember seeing all your fish? Maybe something died during the night and was stuck under a rock? Just an uneducated guess.
What was the temp. I know that I Had a tank in my garage and the power strip failed. So without heat in the great northwest the temp got to around 45 degrees, water was cloudy and well everything was gone. I also know that if all of your parameters are good as you say, maybe there is stray voltage. One of your power heads possibly leaking. Things to check. A faulty heater or stray voltage could definately ruin a good day.
Perhaps something in your home caused the reaction in your tank. Nitrate will not cause what you've described, as I purchased a used set up that was measuring 200+ ppm of Nitrate. Something came in contact with the water, I'd guess.
What exactly did your water test at? When I hear,"All tests were good", it often (but not always) means there's no or little testing.
Hey.. Sorry for the losses. Can you describe your tank? Equipment that you use ? How long it has been up? May help some of the pros here to diagnose what may have caused this