I agree with sphelps.
If you had a sudden total fish loss after simply adding a fish it is likely that for some reason the temperature got too high or too low.
If your tank is small and the oxygen level borderline, adding a tang, other than a small one would do it.
What was the water temperature when you discovered the loss?
Do you have critters in your tank that would set off toxins? Boxfish, sea apple etc.
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