Formula Water

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LO is a month old now so I probably should have asked sooner but - what type of water are you using to mix up formula?
My pediatrician said it was ok to use bottled water so I've been using Poland Spring or Infant water to mix the formula. I haven't been boiling the water but recently noticed that the infant water says "not sterile" on it. My mom mentioned she used to boil water and kept it in a Thermos for our formula.
Have I been doing something wrong? Can it affect my LO? He's been fine all this time...
Help? TIA.