I dont think you’ve said how big the tank is, (forgive me if I missed it) but if it’s reasonably small why not empty it, get new saltwater and start again rather than battling problems from the start ?
The LFS should hold this fish for you until you sort the water out, or if the tank is small enough maybe you can do it easily anyway.
Once the water is up to temperature and salinity, add the ATM colony if you have enough left or get some more, in the correct dosage and then add the fish and just feed lightly that’s it, and no ammonia issues. Just follow the instructions.
The problem is, you have ammonia so your doing water changes, but if you’ve used a bottled bacteria you’re removing it with the water changes (because it’s not established in any rockwork for example and still floating in the water column) so.....