Why does water quality matter so much for espresso?

Water makes up 90%+ of your espresso. Water that's too soft (low mineral content) is corrosive and under-extracts. Water that's too hard builds limescale in your machine and over-extracts bitter compounds. The SCA recommends water with 75–150 ppm TDS, low chlorine, and near-neutral pH. Hard tap water is the most common cause of premature machine failure — it's worth addressing with filtered water rather than hoping for the best.