I know John (ShawFamilyof4) used them when he unwittingly put a brass gate valve on his tank.
They do work. But from my recollection it may also remove other metals as well (like trace elements). If you are worried about getting copper and stuff in your tank from your make-up water then buy a holding container and use the pads in the holding container to remove metals prior to being used in the tank. It's much better to treat anything prior to making it to the tank than it is to try and cure the problem within the system.
Great alternative advice !!! Ill have to figure out how to since I use gallon jugs to mix up mine rather than a bucket. I can find something to add it into that will fit in the opening. But Great Idea!