Substrate Heaters are also known as Undergravel Heaters. You will generally find that these heaters are used mainly by enthusiasts with large planted aquariums.
As the name suggests, these heaters are placed in the substrate or gravel used in your fish tank. This makes sure that the entire tank gets heated and not just above the gravel.
These are normally low powered compared to normal heaters and provide gentle heating of the substrate. Ideally these heaters are used together with one of the more conventional heaters to get the fish tank heated equally.
Undergravel heaters are made up of a heater wire, or grid, which is placed under the gravel. The wire itself is what heats the gravel, it is also said that the substrate is able to retain the heat.
These heaters have specific advantages for planted aquariums. The gentle heating under the substrate aids circulation of water, which is good for plant roots. Without the heating under the gravel, the water there would not circulate.
The circulation brings fresh water and nutrients under the gravel, as well as removing certain toxins which would normally be trapped there. These factors, together with the fact that the roots are also kept warm, all promote good plant growth.
So now you know a bit more about these heaters you will be able to decide if you need one of them. If you have a planted aquarium but are struggling to get your plants to flourish then this may be something you want to look into.