If you were running the tank empty for two weeks, were you "feeding" your bacteria with ammonia? If not, there would be a significant die-off or at least "die-back" during that time which would result in partial loss of cycle; adding a large fish then would inevitably spike your ammonia. The tank would regain its equilibrium much faster than a brand new one, but a blip is going to happen.
I'd be watching the levels like a hawk until they drop, and would expect that some water changes will be needed to keep the fish comfortable during this period. Minimal or no feeding for the next few days or couple weeks. I don't have much experience with water conditioners (I'm on a well) or bottled bacteria, but apparently Prime helps temporarily de-toxify the ammonia? We need an MFK chemist to weight in here...
copy that, thanks.