Test your source water for ammonia.
Why are you using Amquel? Do treat tap water for chlorine? Or a ammonia-preventitive? What test kit are you using for ammonia?
What happens with Amquel, is that it does breakdown ammonia (NH3), into ammonium (NH4+), that is, somewhat, safer for fish. What your test kit (probably API kit?) is reading total ammonia, which does include NH4+. So the ammonia has been "locked" away, but your test kit will still read it. To test for the actual ammonia in your tank, you'll need a test kit that can read for "free ammonia." I use SeaChem kits.
If you are doing 10-15gal daily changes on a 55gal with only a Coral Beauty in there, I really have my doubts ammonia is hovering at .5ppm.