I would have never guessed! Is there other things to test with?Per @Randy Holmes-Farley the Hanna Copper Checker is not accurate at low ranges. It's accurate around the 2.25 ppm therapeutic dose of copper needed to kill Ich, for example, but the Checker shouldn't be trusted at very low levels.
Here's just one thread where RHF mentioned this:


Move what you can down,turn your light percentage up a bit and leave it alone ,do a water change, and spectate. Was it started with all new rock and substrate? What or how did you even get involved in a copper tester to begin with? And some Hanna tests require pressing or press and hold for some actions, just be aware of that on the various testers. Remember to be as consistent as possible in your water changes meaning duration and volume. Marine tanks love stability in every way