you cant use them to heat a tank but they are mainly for plants as they heat the substrate up which promotes root growth. Also because of the heat which rises, it creates a circular motin of water which means no anaerobic conditions.
i copied this of a website;
Undergravel Heaters and Substrate Heating
Substrate heating is accomplished by using a cable laid out in a snake like pattern on the bottom of the tank, beneath the substrate. The cable is laid down inside the tank itself. Undergravel cable heaters are wonderful for plants, and is their number one reason for being used. Using substrate heating has many beneficial effects on plant life by creating active circulation through the gravel with the extra bonus of not being seen by the observer. The heating process involves conduction. The heat from the cable will emanate into the gravel, and from the gravel into the water. The primary heating process though is convection. Having the cable spread appropriately on the bottom will create hot 'pockets' of heat, along with cool pockets in between. This causes the hot water pockets to expand and rise to the surface. The void is filled with water from the cool pockets. The void left by the cool pocket water is filled by sucking water from above the substrate. Convection can remove harmful substances out of the substrate and will oxidize the environment, keeping trace elements in their reduced state. Plant life utilizes trace elements in their reduced state, but has difficulty feeding from oxidized elements. Substrate heating will also introduce (push) healthy nutrients into the substrate, giving the plants easy access to feed on. Because of the necessity in removing the substrate to adjust the heating cable, substrate heating should only be utilized if the fish tank setup is planned to last for at least a year.
