Originally Posted by maddad
Why not consider RO or bottled water the first time around?
That's what I do. Distilled water and add minerals as needed. No testing required because you know that the levels of everything will be 0ppm anyways, and you can add your own salts to get the EXACT mineral profile you want.
And as AJ said, if you really want to get into the water science, you need some rather expensive equipment and supplies to check your water every single time you brew, to account for possible changes. But, despite the cost of buying bottled water, simply building your water from distilled is probably cheaper for most homebrewers - and you don't have the headaches of dealing with too much of certain minerals.
Of course, if you're a professional brewery or otherwise making enormous batches, using distilled water for brewing is neither cheap nor practical, and the proper equipment for testing all relevant aspects of the water becomes a smaller proportional cost and the much more reasonable option.