Depending on how big your tank is and the temp of the room will determine if the heat mat would be adequate. The problem with some of the heat mats are the temperatures range, so make sure you have a thermostat.

Either way place the heat mat on the outside of the tank, preferably on the bottom of the tank.

You can put the plants in the tank, but just be warned everything you put in the tank you must clean thoroughly which can be hassle.