Do you have the tank jacked up at all or is the pad flush against the bottom of the tank? If there is not enough "breathing room" between the enclosure and the surface it is sitting on heat will build up under there fast. Try some rubber feet, drawer knobs (glue-gunned on) or anything else fairly stable you can to ensure there is a venting area under the enclosure of about 1/8 - 1/4". You definitely dont want the base surface in the enclosure anywhere near that temp.

You also might want to think about an inline dimmer switch so you have a lot more control of the heat output from the pad.