Substrate Heating

I was reading the archives about substrate heaters, and thought of
something that seems revolutionary to me, but may have already been
considered and dismissed.  The use of such heaters is to keep the
substrate the same temp as the rest of the tank, right?  Well, I had
two ideas:

1.  Why not run some sort of tubing (I would imagine the smaller, the
better) beneath the substrate, and pump the aquarium water through it?
I'm imagining mounting a large coil of such tubing on a sheet of
acrylic the size of the aquarium bottom, and putting the substrate on
top of it.

2.  What about the undertank heaters used for reptiles?  I would think
that heating a tank with these alone would burn them out, but wouldn't
they serve to keep the substrate at or about the tank temperature if a
standard aquarium heater was used as well?

BTW..  Much of this is assuming that the substrate should be the SAME
temperature as the water, not WARMER?  Even if it should be warmer,
wouldn't the same be better than room temperature?

Brian T. Forsythe
crom at cris_com