Under-tank heating coils.

I'm sorry, but I can't even come *close* to justifying $874 for
the "Dupla Cable Heat System Complete for 130-200 gallon aquariums"
from Daleco for my new (and empty) 180 gallon tank.  There has
got to be a better way!

I'm sold on the concept of a good substrate with heating coils, and
think I want to go with laterite in a setup similar to "The Optimum
Aquarium".  I suppose I will just have to pay more attention to the
DIY heat coil options.  I can't imagine 12 gauge wire, a transformer,
and a few temperature probes running me that much money.  I can coat
everything with latex rubber to help for a good seal ($8 a pint),
but I don't picture that much abuse to the coils in the substrate.
Even if I do get a leak in my wire seals, the fish aren't in danger
because of the transformer, and won't the coils even continue working?
Doesn't the wire offer (much) lower resistance than the water?  We're 
only trying to generate heat here.

I'd like to revive the thought someone posted a week ago about placing
heat coils UNDER the tank, not in the substrate (most aquarium stands
have open tops for good access, especially when you build it yourself :-) ).

Let's refine some logic here:

One issue was that heat coils IN the substrate would provide better
granularity for convection currents for better over-all circulation.
OK, I buy that.  Following this argument, the problem would be more
pronounced with bigger tanks:  You would have more difficulty generating
fine (small) convection currents by placing a heating coil below a
1/2" glass bottom than below a 1/8" glass bottom because more heat would
be distributed over a wider area as it rises (conducts) through the glass.

Now, next point:  What are we after?  Convection.  Slow water movement.

To solve this problem for large tanks (and my particularly empty 180), I am
proposing ALTERNATING heating of the substrate.  If I run a heat coil
BELOW the tank and that warms even a 5" wide area (to pick a number), what 
if I turn it off after a while?  Then I heat the 5" area that had been 
relatively cool, which will then become the source of the convection vent, 
and the first convection vent will be engaged in down-draft.  The layout 
would be the following:

|  1   2222222222222222222222222222222222222222222222222  |
|  1   2       2       2       2       2       2       2  |
|  1   2   1   2   1   2   1   2   1   2   1   2   1   2  |
|  1   2   1   2   1   2   1   2   1   2   1   2   1   2  |
|  1   2   1   2   1   2   1   2   1   2   1   2   1   2  |
|  1       1       1       1       1       1       1   2  |
|  1111111111111111111111111111111111111111111111111   2  |

Under this system, we would have alternating ebb and flow as water cycles
through the system.  We would not be dependant on one-way convection flow.
Sometimes the convection would go UP over a coil (when it's on), and 
other times the convection would go DOWN over a coil (when it's off).

If this would work on larger tanks, it would certainly work on smaller
tanks with thinner glass bottoms, because the heat dispersion through
conduction as we move up through the glass into the substrate would
be less, and we could get a more narrow convection vent going.  Even if 
we are talking about heating very large areas with one wide convection 
(I can't picture a need to heat more than a 12" wide area, even through
a 1/2" glass bottom), I would still think that this alternating 
convection flow would provide adequate slow-moving circulation for ion 
exchange and gentle oxidation.  It would certainly be a heck of a lot 
cheaper and easier to set up, modify, and maintain, and we wouldn't 
need to deal with more electricity in the water (and more wires up the 
back of the tank).

I'd really like some thoughts on this.  It seems logical to me, but I
can't tell if I'm being objective when desperately faced with avoiding 
an $874 substrate heating expense.