It isn't just micro-organisms. It is minerals also.
There is also oftentimes a concern for the lack of buffers for GH/KH which means the PH can swing quite a bit. But if you are getting less dead fish now then before this probably isn't a big problem in your tank.
Maybe some others can give you better feedback on that one. Not sure how much would be good to mix if your tap water was that bad. Personally I haven't had any trouble with using tap water.
There is also oftentimes a concern for the lack of buffers for GH/KH which means the PH can swing quite a bit. But if you are getting less dead fish now then before this probably isn't a big problem in your tank.
Maybe some others can give you better feedback on that one. Not sure how much would be good to mix if your tap water was that bad. Personally I haven't had any trouble with using tap water.