What type of thermometer do you have? Is it the floating kind? Or the kind in a metal/plastic holder?

I had one in a metal holder once that broke at the top. The actual thermometer slid down, so it was telling me that my tank was only about 65 f (the numbers were on the metal), when in reality it was about 95f. I cooked all my fish. I was devestated when I discovered the problem.