When you refill a water bath, you need water that is not too salty, not too pure, but just right. Tap water is usually too impure and will lead to the build up of scale, and in some cases, chlorine corrosion. At the other extreme, the ultra-pure, deionized water from laboratory purification units (e.g. MilliQ or Barnsted ultrapure water) can promote corrosion, even for stainless steel. Instead, you should use a purified water that falls somewhere in between, such as singly distilled water, reverse-osmosis purified water, or whichever kind of purified water is plumbed into your building.