Just to add on a bit, CO2 will speciate differently depending on the pH of the water. H2CO3, HCO3, and CO3 will all form as a function of water pH and the equilibrium constants of those species in water. Mineral water is not a completely chemically defined solution but it typically has a high alkalinity, meaning a large amount of acid can be added to the solution with only a small change in pH. If the starting pH of the water is high, the addition of CO2 will only drop the pH slightly, and it will still be bitter.