Sorry for the late reply....
The ammonia issue can be linked to the copper use, either the biofilter is degraded or the ammonia test kit is being affected by the copper. Which ammonia kit are you using and how are you performing the test? If available to you, Seachems ammonia alert badge works well and helps prevent false readings.
As far as when you should do a water change, anything above 0.1 NH3
means a good sized water change. Also ensure you dose the new SW
with copper before doing the water change. This helps keep the copper concentration at optimal levels rather than diluting the QT
water and trying to "guess" how to elevate it after the fact.
RE: the copper level, what test kit are you using? Coppersafe should read 1.5-2.0 ppm