Substrate heating in planted tanks is another of those "fads" that come and (generally speaking) "go" after a time. Obviously, they fade away because the benefit is minimal, if there even is one.
The principle behind it is that in natural water systems, water percolates down through the substrate. Due to the bacterial processes ongoing in the substrate, namely the breaking down of organics, heat is produced and this warms the water. The water then rises (as warm water and warm air do), back up into the water column. The cycle continues. The benefit of this is that the cooler water brings with it the nutrients plants need at their roots.
This will occur in any planted aquarium, much as in nature. Heating cables on the bottom of the tank under the substrate were intended to quicken this water circulation, thus improving plant growth by increasing the nutrient supply. While this seems logical, the idea never really caught on at least for "natural" or low-tech systems, which suggests to me that the benefit is not significant. Hiscock mentions that in tanks where thin layers of nutrient-rich substrates are used, heating cables underneath will improve nutrient flow; but "substrate heating is not a vital part of a good planting substrate."
Most will agree that heating the substrate is only beneficial in high-tech setups using CO2 diffusion. Walstad for instance mentions that in such systems, allelopathic chemicals released by plants can very quickly build up in the substrate, and after about a year the plants usually begin to fail. Heating cables prevent this by moving the water faster, thus "washing out" the alleopathic chemicals. All substrates lose their nutrient value in time, and nutrients must continually be replaced no matter what the media. In tanks with CO2, more nutrients are needed to sustain plant growth, and here the increased water flow would benefit.
Last edited by Byron; 04-04-2013 at 12:40 PM..