I used distilled water from the market to mix formula on the belief that it was free of contaminants. It is not like the small bottles of "sterile water" that you see in the hospital with a little strip of paper over the top that are used in procedures. However, distilled water is steam-distilled, not just filtered, so it is free of bacterias, contaminants from the local water system, heavy metals, and so on. Incidentally, I didn't use Nursery Water for anything -- it has added elements that double up on things that are in formula in an undesirable way. (I think it is for babies who need to be given a bottle of water, not for use in mixing formula.)
My doctor actually told me to give my son water in between bottles. I almost fell over after having my daughter just two years ago I was told Water was a huge NO NO, he is the same doctor. He said my son was eating too much and it is justified to give water if the infant is eating too much. That was when he told me that the nursery water was not good for babies. I am shocked they are allowed to even be on the market. He said I had to boil water, etc. before giving it to him and that is really hard to do with two babies at home I just dont have the time so I bought ready to feed formula for him and only boil the water I give him in his bottles.
Hopefully moms reading this will now be aware.