Replied by Ted (Bangkok, Thailand)
The biggest problem I encountered came from high heavy metals such as lead, mercury, cadmium, copper, zinc that seems to leeched from old pipes. The second problem is the chlorination and fluoridation. Hence, heavy metals, chlorination and fluoridation should be removed. Removing chlorine is easy, I can't say much for fluoride, but heavy metals are reduce mostly (sometimes not acceptable) from using R.O. water, apparently a bad R.O. water can further introduce contamination if the chrome inside the filtering device are not smoothed, but very rough, increasing the surface area for metal contamination.
Distilled water, where remineralization by addition of sea salt and baking soda seems to be most viable solution that I can think of.