I'm not trying to be an expert on this, just going by what I've seen, but usually it is the minerals in water that causes it and all tap water has minerals which vary from one city to the next (ever notice how water taste different everywhere you go?)
So maybe if you are going to soak your Reo for a long period of time you should use distilled water. I think most people rinse and dry right away which doesn't give the minerals time to do damage. Just a thought.