Yes, I think you are right on target there. The high magnesium content was always part of the debate here when it was discussed the quality of tap water over the years I was in the hobby. Generally, I don't have to much trouble growing plants, but it does take a lot of effort to dissolve CO2 little higher than usual and to be very elaborate with getting it around the tank.
Perhaps related to this story, very close to us, across the border in Slovenia, they have the highest Mg content carbonated bottled water anywhere I guess. It is called Donat Mg and it has 1 gram of Magnesium Carbonates per liter. One 2 dl glass is usually enough to rectify any constipation issues one may have....