Related question, how much does a municipal water supply's mineral content vary from month to month? Say it was coming from a river, I assume this would be more subject to seasonal variation than a reservoir or aquifer.
Oscar, as a first step you might try adding a 1/2tsp of calcium chloride of sulfate and look at how that changes your breaks. If you see an improvement, you probably do need to adjust your water. also, based on knowing if your water is relatively hard or not, you can make some initial assumptions about what you'll need to add.