Exo Terra substrate heater question.

[airman_miller]

I recently purchased a substrate heater for my tank. the tank is a 75 gallon glass aquarium(nothing in it as of yet) and the pad is a large(50-60 gallon i believe). the problem im having is when i went to put the heater on it touches the plastic border of the aquarium. is this a problem? should i get a smaller size? also if i get the dumb thing on there ventilation will not be a problem, as the entire bottom is open to the air.