I would like to add a little bit to this. I would get 2 thermometers from wal mart (accurites work well) and put one probe on the hottest part of the glass and the unit on the hot side. Take the other thermometer, put the probe in the middle of the tank and the unit on the cool side.
This will give you all the temperature readings you require. Also if you don't have a thermostat. Get one.