I have a 4 months female Leopard gecko (5,5"-6" long) in an Exo Terra Terrarium 18"X18"X12". Can this terrarium still be good for an adult gecko. I use an 11"X11" heating mat 14W bought yesterday (the thermostat still hasn't arrived), yesterday evening I've managed to reach 75-77 °C on the "hot zone", but today he didn't work. Could the Gecko survive temperature lower than 64 °F and higher than 91 °F? Is the mat too big and too weak? My gecko pass most of the time in the den of the "hot zone", but today I've managed to feed it.