Mixing metals in a loop leads to galvanic corrosion taking place on your metallic curfaces over time unless you use a corrosion inhibitor. It's bad news. It's not simply staining. It will seriously and magnificently screw up your blocks, rad, pump... Basically all the stuff that has metal surfaces in contact with your fluid. Again, a corrosion inhibitor additive will take care of this problem.<br />
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On another subject, mostly because I don't think anyone has really made this clear... Water is the best coolant you can use in a liquid cooled PC. There is nothing better. Try as you might to fight one that is engineered to perfection, water beats everything hands down.<br />
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So why is it that these coolants exist? A combination of marketing and people who figure that "x coolant is used by NASA on the Space Shuttle!" and therefore that coolant must be more effective than water. They never are. Most specialized coolants exist to perform in circumstances where water can't be used.<br />
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For example, antifreeze (water + propylyene glycol) is used as a coolant in cars because in the winter, you don't want the coolant to be frozen in the lines when the car starts up. Not because it's a more effective coolant. The glycol dilutes the water's effectiveness as a coolant but it's used because the circumstances of use require it.<br />
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These special circumstances don't exist in PC cooling. Your water will be between 15c and 60c regardless of what you do, and this range doesn't require special additives. You are better off using straight water with a tiny bit of some biocide to make sure microbes don't grow in your loop, and that is all.<br />
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So, to reiterate. This isn't just people telling you to save your money and "settle" for water. They're telling you to use water because it is better than whatever x coolant you're proposing, because we've seen them all. They never stack up to natural H2O.
