flouride

It could likely be replaced with hydroxyapatite instead (it also can be used to remove lead and other things from water, which makes searching about being added to municipal water difficult). Good for not only teeth, also bones.

I also wonder if adding other vitamins would make more sense (just enough to stop deficiencies) if we're talking about health outcomes, though the first idea I had with vitamin C came up with results of that messing with the chlorine in the water.