Chemistry time.
In water purification, there are various classes of electrolyte contamination. Various numerical standards exist for types of purity according to conductivity, but you can best recognize the following, in order of highest to lowest electrolyte (ion) content:
Hard water
Tap water
Soft water
Distilled water
Deionized water (DI)
Ultrapure water
Water in the latter two categories is very caustic because it will leech ions from whatever it contacts. Drinking a large amount of DI, for example, will cause it to pull calcium from your bones and sodium from your blood, making you ill.
DI will do the same thing to a water-cooling system by drawing metal from any aluminum, copper, etc. present. Such a small amount of water will not draw out enough metal to make a perceptable difference in the structure of your components, but it *will* cause the DI to increase its ion content, and thus its conductivity and ability to function as an electrolyte in a galvanic cell, such that it will protect your system no better than distilled water.
Since using DI or using distilled will give you the same result, use the cheaper one.
Alchemy
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