The probe for the zilla thermostat is way to big to place between tank and mat. Get aluminum tape and tape the pad to the bottom of the tank and tape the probe to the mat it self. Use aluminum foil tape.
run the thermometer inside the tank,dead center of the heat pad. What I did recently,is went out and bought 1/2 length of PVC pipe and 2 90 degree elbows. Cut a piece the height of the tank,then place the elbow on,the cut another piece,the right length so he hen u run the thermometer probe it just makes out to the center of the mat...I'd take a SMALL ,very small piece of aluminum foil tape,make sure the surface is clean dry and tape the probe to the glass. Then cover with substrate. That's how I got my set up and my thermometer probe is very accurate to what my thermostat reads...
the 6' length of PVC and 2 elbows cost me about $7.00