So a lab test of both water sources will tell you what is already in the water, as there are nutrients in your water that you can use.
For example:
It might tell you that your tap water source is high in calcium but not magnesium. In that case, you might have to supplement magnesium sulfate(epsom salt) instead of using a CalMag blend.
It might tell you that your spring water is high in iron and manganese. Both are essential micronutrients. And in a well or spring water source, the presence of manganese is commonly found with iron. While high iron content is not usually a problem, a manganese toxicity can cause what is called chlorosis.