water would be the coolant. Hmm, H2O2. I am not too sure, the peroxide ion is VERY unstable, and will easily break up into H2, H20, and O2, putting air in your system, and wearing itself out over time. I am thinking along the lines of some the anti-algea liquids they use in aquariums to keep algea from growing. HCL would work as it does not eat away copper, but it will react with aluminum and maybe with brass, so it probably wouldn't work. I am going to have to think about it, so far, the anti-algea stuff sounds the best.