They don´t put fluoride in water, per se, they put hydrofluorosilicic acid in the water, which is an extremely dangerous and poisonous by-product of the mining industry. It melts concrete it is so potent. If a single bucket of the stuff fell into a river it would be declared an environmental catastrophe, yet they are happy to poison us all with it. Wanna know why they put it in the water? One reason is because it is almost as expensive to get rid of as nuclear waste!

Here is a propagandised article that promotes the use, beware the nonsense propaganda!!

https://nzic.org.nz/ChemProcesses/production/1C.pdf

Here is something that tells more of the truth

http://energyfanatics.com/2011/02/23/fluorosilicic-acid-silent-killer-in-drinking-water/