Maybe not everybody knows that Rome has a very efficient system of running fresh water that the city inherited from the ancient Romans.
The Roman aqueducts are still one of the marvels the ancient Romans have left, not only in Rome but also in many other regions of what once was the Roman Empire.
What is really astonishing is the fact that the city of Rome still relies on the lines mapped by the ancient Romans. As a result of that, the city have freshwater arriving constantly from many springs located in the Apennine mountains that surround the territory of Rome.
So drinkable tap water in Rome is definitely not an issue. There are thousands of small fountain all over the city (the so called “Nasoni”) where people can directly drink, and also many of the monumental fountains are meant for drinking purposes (as for example the “Barcaccia”, a Bernini fountain at the footsteps of the Trinità dei Monti Staircase (in the Spanish Square).
No reason to look for mineral water when in Rome then. Every house has running freshwater, and every corner of the city has a public fountain where to drink.
One think anyway visitors have to consider: never ask for tap water in a restaurant. It is quite annoying for the restaurant owner. Do not ask us why: it is like that, and it is like that if you are an Italian, American, Australian or Roman client.