I've been using paint pens for years to mark my gear and, other than having to occasionally redo the label, I've never noticed a problem. Most of my tanks are painted and there doesn't appear to be any damage to the finish - certainly less damage than happens playing tank skittles on the boat or shore...
The paint pen is also removable, should you ever wish to sell.
The paint pen is also removable, should you ever wish to sell.