Biologically inert water mixture (or other fluid) for heat-exchange purposes
Apparently an acceptable choice of additive is silverised hydrogen peroxide. Only a few 100ppm should be necessary to keep the water sterile and the half life is about 5 weeks. 

I'll go with this and update this answer if there are any issues.

PS

This article on overclocking (http://www.overclockers.com/pc-water-coolant-chemistry-part-ii/) has a number of suggestion on water additives to improve the thermal conductivity and  keep the system clean.