Water is the vehicle for everything the plant takes up. Its starting quality determines what you can do with your nutrient solution, and the way you measure it decides whether you can diagnose a problem before it costs you a harvest. This step covers raw water, the solution you make from it, and the two measurements that drive everything.
Your starting water, before any additions
Two properties of your raw water decide how much room you have to manoeuvre, and they are not the same thing.
Hardness is the amount of dissolved calcium and magnesium. Alkalinity is the water's buffering power, its capacity to resist a change in pH. Highly alkaline water stubbornly pulls its pH back up no matter how much acid you add, and that behaviour costs you in consumables and in instability.
A water analysis costs little and only needs asking for once. It tells you what your solution already contains before the slightest addition, which changes the calculation entirely: adding calcium makes little sense in water that is already saturated with it, and a great deal of sense in reverse-osmosis water.
Going with reverse osmosis resets the counter to zero and makes everything you add fully controllable. That is real convenience, at the cost of an investment, of waste water consumption, and of the obligation to put back everything you removed, calcium and magnesium included.
