automotive coolant/antifreeze is a fairly common additive in H2O systems but there are some others that might give a little better performance. Straight distilled water has a little better performance than the 2/3 &1/3 mixture that you are using but it also has problems with stuff growing in it or heavy galvanic corrosion if aluminum and copper are used in the same loop. Other than those, water wetter is used quite a bit along with others such as Zerex racing coolant. Of course, there are also number of UV reactive dyes that are added so the water will glow when a UV cathode is added to the case.
The latest addition to coolant types are the ones like Fluid XP. While they are expensive, they are also dielectric (meaning they do not conduct electricity). From the info I have seen, Fluid XP doesn’t cool quite as well as pure distilled water but it won’t fry anything if it leaks on your vid card or board. Non-conductive liquids have been around forever but the cost has kept them out of the watercooling market until recently. 3M still makes a line of nonconductive liquids for various uses but some of them are over $100 per gallon…
