Rust occurs when moisture and air come in contact with ferrous material. The purest water in the world will cause rust. It has nothing to do with the purity of water. Dogs will drink toilet water, so I find it ironic that anybody would worry about tap water. Also be very careful about the amount of faith you put in those charcoal filter systems or even bottled water. Tests have shown in most cases it is not any better than the average tap water. When your dog dies it won't be because he drank tap water.