In recent years there has been much fuss concerning substrate heating. My first interest in substrate heating stemmed from my love of catfish. I was concerned that catfish like to rest on the glass immersion heaters that have become so popular and fish, being poikilothermic, that is cold-blooded, cannot sense when they are being burnt by the heaters. My prized catfish frequently displayed white burn marks that some times never fully healed. The solution was to hide the heater in some way, but this often lead to poor heat distribution or the use of unsightly heater guards, or to remove the heat source from the aquarium.

I found that some aquarists were heating the aquarium by using a heating cable buried in the substrate or an external heat mat similar to those used in terrariums. Heat mats and cables provide good heat distribution since the heating element is spread across the entire base of the tank and the fact that they were hidden, solved the "burning fish" problem. However, I also discovered that heat cables in particular were being used to stimulate and promote lush plant growth.

In nature, plants thrive in thick mud that is all but void of oxygen. If excess oxygen is present in the substrate, it will oxidise nutrients and turn them into non-water soluble compounds that cannot be absorbed by the plant roots. The oxygen that plants require is mainly absorbed through their leaves and transported down to the roots. The substrate in rivers and streams is often fed by underground springs or consists of aquifers - rock or soil that enable underground water to move to the surface and warm surface water to ebb into the substrate. These actions slowly circulate nutrients from within the substrate and the water to the plant's roots and carry away exhausted or waste compounds.

The convection currents created by substrate heating power this gentle flow in the aquarium. However, this flow also conveys some oxygen from the main body of the aquarium into the substrate. The flow of water must therefore be slow enough to allow the micro-organisms within the substrate to consume all the oxygen. This in effect creates a very efficient biological filter within the substrate that requires very little maintenance. To maintain the effect, water within the substrate should only be exchanged approximately once every 24 hours. An over powered heat mat or cable that becomes too warm may draw water too quickly past the plants roots introducing excess oxygen and effectively stripping away the very nutrients that the convection currents they create are intended to carry.

This is why some plants fail spectacularly when an under gravel filter (UGF) or reverse flow UGF is employed. Although these filters permit warm water to flow through the substrate, the current is far too fast. Flow rates on reverse flow UGFs can be restricted substantially but the very nature of these filters limits plant growth. The water passing through the substrate will have just been filtered and nutrients thus oxidised and therefore water will be all but void of the beneficial nutrients necessary for plant growth.

In an aquarium with no substrate heating, water does not flow particularly well through the substrate especially if you have the smaller, 2-3mm gravel recommended for plants. It is therefore usually a degree or two cooler than the main body of the aquarium and this can inhibit plant growth. Heating the substrate provides plant roots with "warm feet" encouraging growth. If the substrate is too cold, the plants can become stunted.

Substrate heaters are often, but not exclusively, designed only to warm the substrate to promote plant growth and therefore are not of sufficient power to heat the aquarium water as well. Usually they are used in-conjunction with an additional source such as a thermofilter to heat the aquarium water. If using an additional heater to supplement the substrate heating, set the temperature of it to around two degrees Fahrenheit lower than that of the desired water temperature. If the substrate heater is not capable of heating the whole aquarium, the additional heater will then "kick in".

It is important to choose carefully the heat mat or cable you intend to install. Attempting to heat the aquarium water an over powered cable or heat mat can have two drawbacks. Firstly, hot spots on the base of the aquarium can be created, particularly when compact materials such as sand or large stones are used as part of the substrate. These items heat up, do not dissipate heat well, and can cause the base of the aquarium to crack. Additionally, as explained earlier, the water flow throughout the substrate may be too great.

Installing these heating devices is only practical when setting up a tank and not worth the effort in an established aquarium. I like the fact that these heaters are hidden and do not influence the fish's habitat. They do have one major disadvantage, once installed they cannot be removed without major upheaval, and should they fail the simplest solution is to leave them in place and install a glass immersion heater or a thermofilter instead.