The Inq heard about a new fluid called Galden at Semicon West 2006:
The company that was showing it off, the Kurt J. Lesker Company also sells it, so mad scientists take note, you can actually buy this stuff. There are a range of fluids in the Galden family, mainly differentiated by boiling and pour points. All are non-toxic and don't burn, a notable drawback compared to one of its competitors.
The working range goes from a high of 250C for the HT270 fluid to a low of -90C or so for HT70, with a pour point of -110C. With this stuff, you can cool your whole board down to almost 100 below, and still see it. CPU and GPU cooling? That is for the uninspired.
So, with this gauntlet thrown down, I should warn you that this stuff is not cheap. It sells for around $350-400 a gallon, but if you want an ultra-cold system in an aquarium, you don't have many choices, much less cheaper ones.