While they sell coolant specifically for water cooling, after my first changeout I simply used the premixed "RV winterizing" antifreeze. It wasn't florescent UV reactive blue anymore, but hey. My biggest concern with using water (of any sort) is that you wouldn't get the corrosion inhibitors.
My current system is watercooled, but that's solely because when I rebuilt it (or more precisely, when I gutted the Socket 939 installation which I put into another aluminum case, and re-used my current case for the new E8400 based system) the motherboard I had chosen already had a watercooling block installed for the chipset, and I already had everything else except the CPU block to do it - and it was already installed on the case. If I had been starting from scratch, there is no way I would have bothered to watercool my current system - I did so originally because I was in a house without central A/C and the AMD's ran very warm under the best of circumstances.
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