Re: 1st Vivarium 36x18x36 Exo Terra
It is hard to heat properly a viv so large. The advice of FrogTim is fine, but I would use a 50 watt or plus aquarium heater.
I do not have water feature in my vivs, so I use under three Exo Terra 12 inches (total 36 inches) one 100 watt heating cable and one t5 (in addition to the Leds). Your viv is bigger, so I would use a 150 watt cable and T8.