bottled water

Im a ftm. My child's previous dr said bottled water is fine. So, since about 4 weeks old i've used bottled water for formula and not infant water. But I'm reading online that It Isn't because it contains a lot of salt. My baby will be 4 months next week. Would something like this cause him to gain a lot of weight? I'm worried now that I'm doing something wrong. But I had no idea! :,(