I have been slowly working on making a temperate marine aquarium w/ species and creatures nearby ( in western washington area ) . And I just had a quick thought..

I read somewhere that when setting up a marine temperate system.. its often useful to crank up the temperature a little bit for the first week or two (again this is temperate, so it would usually be chilled/cold water), thus clearly 'cranking up' does not mean boil or anything close lol. The reasons for this recommendation were namely that this would help allow the live sand and live rock, and whatever else to establish themselves. (oh & this is done before any other creatures are intentionally added). After everything is established the water would be put back to a normal temperature.

but just a few moments ago I was reading something about reptiles and putting heating blanket under the cage to keep them warm...
AH HA! and so it hit me... what if a heating blanket was used under the tank?.. supposing my home-brew chiller does a good job, it should be possible to actually keep a slight temperature gradient in the sand on the bottom, while keeping the water temperature stable.

why would I want this: I would think maybe this would keep the live sand and its ''stuff'' more active, in a tank that I want to be as self sufficient as possible, with a thick layer of live sand, perhaps this would increase filtering capabilities. ?

so what are your thoughts ?

Depending on where you intend to collect animals along Washington state in reference to the subterrainian thermal jacket, water temps there vary from approximately 56 degrees F to 100 plus degrees F. With the temperate area still above 50 degrees F I don't see any reason to try and mix sub terrain heat with temperate water. Although you may get an increase in bacterium activity with the warmer environment possible exposure to rapid temp changes from sand movement I believe would cause dieoff. Inso removing the advantage and possibly creating other issues.
Also having to have equipment that are working opposite of each other could be a problem if one happens to quit.
That being said areas along Washington state up into Canada have water that is heated through sub terrain thermo jackets and I have never seen anyone try to duplicate this biosphere.
Good luck and Happy Reefing

Thanks for your insight !! i didn't know how much the sub terrain was heated to begin with... I'm betting I'd have to have a pretty thick layer of terrain (which I had already thought best to do)... That is particularly interesting since I am hoping to create a self sustaining system. But still in practice perhaps my setup would be too small for this implementation to work properly... I will continue to dig deeper into the idea.

Once again-- Thanks for your response ! while I've frequently made use of reading forums for all sorts of things... this is the first I've actually started a thread, on any forum, and had a response... so thanks!