BrewnWKopperKat
(Ο_Ο)
The extract manufacturer presumably used water with minerals -- certain levels of sodium, magnesium, chloride, sulfate, calcium, and bicarbonate.
You don't know what the levels are and you can't reduce whatever minerals are there.
This is well known. "Adding brewing Salts to Season Your Beer" in chapter 8 ("Water for Extract Brewing") in How to Brew, 4e offers a number of ideas on how to approach brewing salts with extract.
Granted, I can't change the mineral composition provided by the extract (or the mineral composition in the base malts), but I can change the mineral composition of my wort.