This is the easiest and most common method used to reduce caries in a community level or as among general population.
Fluoride in water in ppm — 1 ppm
Optimum water fluoridation in mg — 0.5 – 0.8 mg/litre
Below optimum- 0.7mm
Below optimum = 0.7 ppm in warmer climates
Above optimum = 1.2 ppm in colder climates
If there’s no community water fluoridation, then next cost effective method for 6-12yr old, is to go for school water fluoridation instead of F tablets, pastes, gel, mouthwash etc.