You can't really add a bunch of fluorine ions to water because they'd all be negatively charged. We say we're adding "fluoride", but really we're adding ionic compounds that include fluoride.

This seems analogous to the difference between chlorinated water (toxic) and salt water (not at all toxic). It's always interesting to me that adding chlorine to water makes it poisonous, and adding sodium causes it to explode, but adding sodium chloride does... nothing in particular.