I needed to leave my pool unattended for a month. It's leaf season, so I decided to disable the pump and leave it dormant as there would be nobody around to empty the skimmer etc. every day. As an "extra precaution", I dropped in 10 or so chlorine tablets at various locations around the pool. I got back to find all OK, except that I now have a big nasty brown stain around the remains of each chlorine tablet. Like the chlorine tablets were big rusty bolts or something. I'm suprised by that ?!? I've heard of using chlorine tablets to remove stains, but not to cause them. Any idea what's going on ?