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Not everywhere has the same 'tap water'.
In many parts of the western U.S. local water is very hard and will to eaten up aluminum intakes.
I always use distilled water with my coolant.......
tap water, 40 percent prestone green antifreeze, 60 percent water if you live in the south and i run a 12 ounce bottle of flexalite coolant additive that controls corrosion and electrolyisis that eats aluminum.