Well when you do a full water change,should be no ammonia present. Only in their old water,from their waste. I wouldnt trust an old test kit. They do expire after awhile. Go buy a new set,like API freshwater kit. Salt and water conditioner wont affect the ammonia test,unless you use Amquel. That can mess with API test results.
So,if you did a 100% change,no worries. And get a new test kit. lol. They will be fine.