Iron Shavings/Heating Pads

  From what I've read, the main benefit of heating the substrate
is to lead to increased growth of the plant roots and bacterial 
flora as a result of increased substrate temperature.  Properly
set up, heating cables give the added benefit of convection 
currents in the substrate, thereby reintroducing depleted minerals
and nutrients to the substrate.  UGF with small heaters in the
uplift tubes also cause slow circulation, though on a less uniform
	 Substrate additives such as laterite, vermiculite, and peat 
supply trace minerals such as iron and/or increase the substrate's
ability to sequester essential charged ions.  Iron seems to be
of particular importance.
		The exchange properties (sorry I forgot the technical term) of 
vermiculite are particularly good and it doesn't decompose so adding
it to the substrate makes sense to me.  From most of the postings
I've read, iron and CO2 seem to be limiting factors in aquarium
plant growth. The main benefit of laterite is its iron content. 
Wouldn't it be much cheaper to add fine iron shavings to the bottom
layer of the substrate as a source of iron? At least to me, this 
seems to be a long lasting source of iron that would be available 
to the plants but not to algae.  It would reach some sort of 
equilibrium between Fe and its oxidized states (Fe2+, Fe3+).
This would partly remove the need for exchanging currents in the
substrate. Or would it just rust and foul the water?
  Heating pads under the tank would be a relatively inexpensive
way to heat the substrate.  CO2 supplementation would complete 
the picture.
		How important is supplementation of other trace minerals?  What
are the effects of only adding supplements with water changes?
Could vermiculite be soaked in a more concentrated solution of 
ion supplements prior to setting up the tank, producing a relatively
stable and longer lasting internal source of these trace elements?

							Please, let the critique and trashing begin.  After 
all, isn't it easier to learn from other people's failures 
and/or brilliant ideas than by going through the trouble of 
trying it yourself?  

																								Marc Chodos choman at ucla_edu
(in LA, where it got so cold last week that I had to put on
long pants one evening).