I've read that the rock can absorb/bind the copper, so the amount you think you have in the water for treatment is less than you think. Maybe not an issue if you are constantly testing copper during treatment, but something to keep in mind I guess.
Still using the AquaWiz and I still like it. The motors get some corrosion, which I guess is a common issue, but hasn't effected functionality. I could see you building one of these from scratch, could be a fun project.