I’ve used them in my 65g tank since I first set it up just over a year ago. I’ve never had them change anything except for brown, which is a good thing as it means there is nothing in the tank that I don’t want. My initial couple didn’t last too long which didn’t surprise me as it was a new tank.
Since I started dosing NOPOX and getting on top of my tank stability I am finding they are lasting much longer, which I take as good feedback about my water quality. I probably don’t need it on my tank now but I will keep running it as an insurance policy.
I also use a small piece in my QT/HT and after I had a sick fish which I dosed Cupramine for (removed the Poly Filter whilst dosing) I then put it back in to remove the Copper - worked a treat, turned blue fairly quickly. After a couple of weeks I removed that, put a fresh piece in, and as there was no colour change except light brown, was happy to keep using the QT for new incoming fish.