No fish can adapt to ammonia, it is harmful in any detectable amount. My guess is you are using test strips which are inaccurate as 12ppm ammonia will kill just about anything outside of a snail very quickly. Honestly I am disappointed after reading this thread. I posted early on (post #5) and mentioned I thought your water conditions were likely poor, and it seems they are.
To compound this you have made the same mistake again by purchasing a new tank and stocking it with more fish while having an improperly-sized filter to boot. The water you transferred over contains very little bacteria, while the tank walls of the 10 gallon had a good bit of bacteria that you have now lost. The filter and decorations/gravel will house the majority but you somewhat damaged your cycle by switching tanks.
The ammo-chips and medications can only do so much. Ammo-chips can starve your bacteria so that when the chips wear out you are left with a cycle all over again. If your
LFS is stocking sickly fish you should go somewhere else. There is no way I would ever support a place that cannot properly take care of their fish. At best they are incompetent, at worst they are doing it on purpose to sell medications at the time of sale. Either one is bad.
What you need to do is cut feedings to every other day, do daily large water changes to keep ammonia below 0.5ppm, and do some research on how to properly care for fish. And whatever you do, don't buy anymore fish.
HTH