ATTACHING THE TAG:
Instead of using your nails, try wedging a coin between the coils of the tag's split ring, near the opening. Then, simply twist the coin to widen the gap as you slide the tag onto the collar. After attaching, make sure there isn't a gap left in the split ring; if the coils have been bent out of shape, use pliers to gently squeeze them together. When using a leash, always attach it directly to the collar's D-ring (not to the tag's split rings).
REMOVING OXIDATION:
If your pet's tag is made of brass or copper, it will oxidize and darken to an earthy, rustic color. This is a natural (and usually desirable) characteristic of these metals, but if you would prefer to keep the tag bright & shiny, you can polish it with any cleaner intended for brass or copper jewelry.