Remember, tap water isn't tap water. Hard, soft, clear, or dirty in appearance, it can have as many different chemicals added by a municipality as it has minerals added by time.

Around here, our water flows about 25' underground and is said to have fallen centuries ago and is filtered naturally. I dunno ab the content otherwise, but it does not leave mineral spots on a washed car. In some coastal regions, the sun will dry tap water into hard white spots on your car. In some other cities in the US, tap water is tested as unsafe to drink.

Extremely hard or soft tap water would be very bad for a cooling system.

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