Biologically inert water mixture (or other fluid) for heat-exchange purposes I have a steel chamber submerged in a 14 deg water bath to keep it at constant temperature. The setup needs to operate without intervention for a very long time (weeks or months). 

The bath is exposed to open air in the lab and we have been having issues with mold development in the bath despite adding bleach to the water. 

Can you suggest a different additive (or a different liquid altogether) to use to make sure the bath stays reasonably sterile?
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.