So I'm using paper as a substrate for my baby beardie in their 40 gallon but since it's quite cold at night I was going to put in a heat Matt my problem is should I mount it on the side of the tank or is it suitable to go on the bottom even though I'm using paper as a substrate
How cold is it getting inside the tank? If it stays above about 65 I wouldn't worry about it. I prefer using heat from above via a CHE or a heat projector when needed. The heat mat will heat the surface that's touching it but won't radiate much for the tank as a whole. If you do use one, you'll want to put it on the bottom. Unless it malfunctions, it won't get nearly hot enough to cause a problem with paper. I do advise setting it up so there is airflow underneath the heat mat and using a thermostat for it if you go that route.
How cold is it getting inside the tank? If it stays above about 65 I wouldn't worry about it. I prefer using heat from above via a CHE or a heat projector when needed. The heat mat will heat the surface that's touching it but won't radiate much for the tank as a whole. If you do use one, you'll want to put it on the bottom. Unless it malfunctions, it won't get nearly hot enough to cause a problem with paper. I do advise setting it up so there is airflow underneath the heat mat and using a thermostat for it if you go that route.
OK so that's cold but still well within their tolerance overnight so extra heat probably isn't needed unless it starts to get colder in there overnight. This is what I use for cold nights (and also for my roach colony) and like it a lot http://www.reptileuv.com/shop/index.php?main_page=product_info&products_id=196