Nursery purified water?

Vanessa • Married to the most wonderful & handsome man in the world! 👧🏻💞 born Oct. 23. 2016 & 👶🏽💚 born May 2. 2018 ♥️IG: DwyerBearFamily
What water do you use to mix your formula for your baby? 
We have been using Aquafina but I know they sell water for babies but it has fluoride? Why does it contain this? And what is recommended by doctors? I haven't seen our pediatrician to ask him. 
Thank you!