or do what the nordic countries do, pour some vodka in there. - A bit of alcohol is good for use because it prevents bacteria/algae etc. growing in the washer tank.
tip: - If your tank has a heater like mercedes benz etc, remove the internal coils and replace the O rings that you see on the thermostat on those coils, the coolant leak there deposits salts from the coolant into the water which causes clogging of the system