What they said. If the tap water is hard, it might be marginally better to use distilled water to fill. I should still be O.K., even, to flush with tap water. You will see little problem with water quality in a closed system like this compared to a chilling tower, steam boiler or water heater where the water is constantly being changed. There, corrosion and scaling are huge (expensive) issues.
Even acidic water should not be a problem as it will not have significant buffering capacity compared to fresh coolant. I would not use dog water, use nothing yellow in there unless it comes out of a coolant jug.