First of all, congratulations on your GTS! Fantastic cars
There is no indicator of how much water is left.
The car needs distilled water. I'm sure some setups are capable of producing this at home, but I doubt it's common.
You can try yours and monitor what the stone filter looks like. Even with distilled water, it has some white flaking when we swap ours
In street driving, the car uses very little water
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