Ok, to maintain the correct temps on your hot side, you need the thermostat probe to be on the hot side.

Because I have a rack system that uses heat tape, I can put my probe on the tape itself, but with a heat mat, you'll want the probe inside the tub, under the substrate so it is measuring temps correctly. You can use a small piece of tape to hold it down I'm pretty sure, since the aspen will cover it.

Gale