What is the salt, which when present in excess in drinking water can cause discolouration of teeth?

 Excessive ingestion of fluorides during early childhood may damage tooth-forming cells, leading to a defect in the enamel known as dental fluorosis. Teeth impacted by fluorosis have visible discolouration, ranging from white spots to brown and black stains.

They also show increased porosity of the enamel. In the milder forms, the porosity is mostly limited to the sub-surface enamel, whereas in the more advanced forms the porosity also impacts the surface enamel, resulting in extensive pitting, chipping, fracturing and decay of the teeth.