Fluoride is added to drinking water in the form of:
Fluoride compounds used in water fluoridation

Fluorspar 
Sodium fluoride 
Silicofluoride 
Sodium silicofluoride 
Ammonium silicofluoride 
Hydrofluosilicic acid.

Essentials of preventive and community dentistry
Soben Peter page no. 268
Hydroﬂuosilicic acid
Fluoride ions
APF
None of the above