The source of your municipal water will probably be the source of a high Ca or Mg content. If the catchment is mainly limestone, you'll have a lot of Ca in your tapwater. If your catchment is sandstone, there will be very little Ca & Mg (naturally 'soft' water) and you may have to use Ca & Mg additives to prevent deficiencies.
I have the latter situation, a mainly sandstone catchment. Considering how much I pay for Ca & Mg additives, I wouldn't mind a little 'hard' water!
