I have never kept a snake in a viv/aquarium (assuming you mean the glass tanks, yes?) so I could very well be wrong, but my initial impression is that just a heat mat + thermostat would not be sufficient in that case. I think that most of us who only use a mat/thermostat are using bins or tubs supplemented by ambient heat. Tubs and bins maintain a stable heat gradient that I am not sure can be achieved in a tank.