Well, I guess I have no choice but to write a rebuttal to Tom's post.
To avoid copyright issues, I'll TRY to summarize some of the points and then give my thoughts.
The paper that was referenced does not seem to support any position on heating or pore size of the substrate. The paper referenced is here
To me, what this paper shows is that for the aquatic macrophyte Littorela uniflora, light, root morphology, root density, and sediment reducing capacity all played a role in the oxic zones around the plant's roots. In other words, this plant, like most other aquatic macrophytes, transports oxygen to the substrate via it's roots. This system was impacted by the above factors.
I personally think this papers supports some of the assertions we are making in this post. Please note that this post is not a "Heating cables are great" discussion. The intent is to think through, as Tom suggests, what it is that creates the best environment for our plants.
Tom states that makers of heating cables and substrates that focus on pore size (ADA's Power Sand) state that they will bring circulation into the substrate. This circulation brings in O2 and this will enhance root development. I disagree with this statement.
These products are trying to turn your substrate into a long-term nutrient storehouse for your plants. This is done by providing the right environment for the substrate ecosystem to develop. It is the substrate ecosystem that has a direct impact on the root/plant developement. Bringing O2 to the substrate is not the main objective.
The question that Tom should be asking (or, maybe he did and I just didn't get it) is whether these products assist in creating a better substrate. The answer is not clear but we can make some educated guesses.
Focusing on heating cables, I believe that there has been some discussion by experts in the field that looked at fluid dynamics and modeling to determine whether the cables would create the convection current we spoke of. The answer was "no." So, if we assume this is correct, then we can scratch heating cables as viable products, right?
Well, not necessarily. I would suggest to you that if you live in an environment where you use a heater in your aquarium, heating cables may bring around some benefit. There have been studies done (I don't have the reference handy) indicating that temperature also plays a role in the development of fine root hairs and overall nutrient uptake by aquatic macrophytes. If we could have the cables keep the temp of the substrate at a temp where root development and uptake are optimized, your plants should theoretically do better. How much better? I don't know. Is it enough to justify the added initial and maintenance cost of the cables? Probably not.
Cables came about because of failures Dupla experienced in their early days with one of their large aquariums in Germany. The found that the substrate was the limiting factor in plant growth after a while because it became anoxic thereby cause root rot. Their large aquarium was not sufficiently planted so that the plants themselves could not pump enough O2 into the substrate to keep it from dieing. So, they came up with cables under the theory that they would bring O2 rich water into the substrate therefore keeping it alive.
To be continued...