I concur. I highly doubt it was the tap water that killed everything. I would assume that it would have only created some nuisance algae and introduced some phosphates. In a tank that large, I don't think some tap water would do much of anything. Unless he had major chemical components and treatment in his water supply. But he states that his water out of the tap isn't bad, so this is why I doubt the water.
Now as for the de-chlorinator, that's another story. He would have been better off adding straight tap water than trying to treat it. And would have been better off yet, like he said, just running pumps and letting the new water be made over the 2 day span.
Hindsight is always 20/20 though.
(yiiippeeee 500!!!)