Seems strange that a fish would produce that much waste in such a short period.
In fish shows, it's not uncommon for a 8 or 10" fish to be crammed into a 10g tank with nothing but an uncycled sponge filter for 3 or more days.
First, I don't trust aquarium test kits, especially with water treated with chloramines and/or Prime (or similar products). False results send people into tizzies.
Is it possible that the fish was just stressed out from being a lone fish in a new tank (and not nitrate poisoning)?
You can't do wrong doing a water change but even if the bio media contained no beneficial bacteria (which seems strange, as just about anything in a mature fish tank is covered in the stuff), it would hard to foul the water that fast.
If in doubt, add a sponge filter from a mature tank and some air. And add a couple of dithers. See if that works. And don't feed the fish
Matt