are you sure about the water parameters?
I seriously doubt that adding one fish would tip the balance to the point of everything dying overnight. If so then the water quality was already pretty bad and you should have noticed by the fish's behavior.
It's very unlikely that was the cause.
If the water parameters were good than I would consider some type of contamination as the culprit.
But in my opinion what happened is that for some reason you didn't get accurate test results and your fish died due to high nitrite levels.
If roughly a week + had passed since the transfer and you did move some bacteria over, but probably not enough if you rinsed the sand, (which should have been done in tank water to preserve the bacteria, not fresh water)
, you probably encountered a mini cycle and the timing is just right for elevated nitrite levels.
but even then it is very unusual that everything would die overnight unless levels were rather high, and the fish should have been acting ill before you purchased the tang if that was the case.
You should take a water sample to the lfs