Deionised water is recommended for use in engine cooling systems to offer maximum cooling and minimum corrosion.
The effectiveness of anti-freeze can be reduced by the quality of water used for dilution. If tap water is used in the cooling system, the high levels of calcium and magnesium, can cause deposits and scale build up (this is particularly a problem if you live in a hard water area).
Deionised water is as it sounds: water that’s had the ions removed, using an ion exchange process. Deionised water is low in metal ion content (e.g. iron, copper, manganese) and other inorganic ions (e.g. sulphate, chloride etc) and hence has low conductivity.
Do not store in direct sunlight.