Firstly, raise the tank slightly off the mat so that it doesn't rest directly on it. That prevents the weight of the tank damaging the mat. It also allows airflow over the surface of the mat, which helps prevent overheating.
Most importantly, buy a thermostat. This will regulate the mat's temperature so that it never reaches the danger point where it would get hot enough to break the glass (which would probably be hot enough to kill your Corn anyway).
Plenty of people here use mats with glass tanks - I suspect the place that sold it to you just didn't give you the full info on how to use it. All should be well.