Hmmm. I would start by doing wc's to the tank, and see where it goes. Not really sure if it's mercury or not, I know they make thermometers with mercury, alcohol and some other stuff, not exactly sure what types. I would do some research if I were you! WC is about all I can suggest! Depending on how far along your tank is, I would try changing the sand out as well if you are really worried about it!