If it is copper, I'm not sure the API copper test is reliable at low range values. If there's any chance you can stop by an LFS and grab a salifert copper test it might give more insight. Same thing for a package of poly-pad chemical filter pads.
The snails that are breeding are trochus, I assume?
We have 6 different kinds of snails and at least 3 different kinds are laying eggs based on the eggs we are seeing
We had no intention of putting the fish in yet however the heater died on the QT tank and we lost 3 fish in QT overnight due to the temp drop and the 2 remaining were breathing fast and were lethargic. We moved them as a panic and within a few hours they both stabilized and are seemingly happy. QT tank has now been cleaned and has a new heater so we can start another QT batch.