Good News!
Actually, I have a little practical experience at this, funny story.... (well not funny, more stupid and careless, read on....
Back in my youth (maybe 1983), I was a Fast Food Manager, once an employee was stirring a fryer with one of those huge thermometers. It broke and he didn't tell anyone. Later on that afternoon, after having used the fryer all day, another employee noticed pieces of the thermometer laying in the bottom of said fryer. I had to make frantic phone calls to the manufacturer, and my supervisors thinking we'd Mercury posioned half of Lawrenceville (to say nothing of the glass that was in the fryer). According to the distributors of the cooking Thermomters, "mercury isn't used anymore for anything other than medical grade equipment, as its so expensive, its cost prohibitive". Turns out no one got sick or sued, and that employee was fired.
I'm sure our tank thermometers are probably made with the cheapest metal possible, since you can buy them for a few bucks anywhere.
Cudos to Mojo for thinking up the magnet idea!