How bad is fluoride for babies?

I have a 7 month old and have been using the nursery water that contains fluoride to make her formula since she was three months old. My father has recently informed me that he had read an article about fluoride killing brain cells in babies. What I don't understand is why are these companies making water for babies with fluoride if indeed it is not good for them. How much is ok for babies to ingest? What can I do instead? Can I just use filtered water from the fridge or faucet? Please help!