Cable confusion: what watt?

     I have an order pending with Pet Warehouse for their Dupla
heating cable package including a 200w cable, a Duplamat250
transformer, and a Rocon Digi-stat, though I'm considering
cancelling the order and buying the same set-up from Daleco
except with a 250w cable instead.  (Daleco is a little more
expensive but has more knowledgeable customer service, plus
they have the equipment in stock.)  Incidentally, a Daleco rep
could not tell me any difference between the Rocon Digi-stat
and the Tunze Controller, apart from the higher price of the
latter.  I decided to buy Dupla cables rather than another brand.
     Having studied _The Optimum Aquarium_, the FAQ's, 
several posts from the APD, and more in the Krib archives
(thank you, George Booth and others), I am enlightened as much
as confused.  George Booth wrote that he and Karla bought a
250w cable system for their 90gal, but that they found they
needed to add an Ebo-Jager because their metal halides
increased the water temperature to the point that the cable
would shut off before the lights went out and would not be
able to heat up the water fast enough when it (the cable)
turned on again.  I believe that the water column was kept
at 82F, with a substrate depth of 3.5", and a reading at the
bottom of the substrate of 92F.
     The cable I get will be installed in an All Glass 110gal
x-high (48" x 18" x 30"), which has the same "footprint" as
the Booth tank mentioned above.  (You don't have to remind me
what a pain it will be to have to be reaching down to the
bottom of this tank to replant stems, etc.)  It will be kept
at 82F (discus), and I have been using two 150w heaters on
it--it's a bare tank now.  It has two 250w, 5500K mh
pendants suspended 17" from the water surface (yes, I believe
this will provide sufficient light for java fern to photo-
synthesize <g> ); furthermore, a fully automated CO2 system,
but, alas, no trickle filter:  it's an Eheim 2260, the hugest
canister filter I've ever heard of (I know, I know, a lot of
work to keep clean).
     Now to the point:  since I expect to have to use an auxilliary
conventional heater anyway, it seems that higher watt cables
are a waste of money, since lower watt ones, within 
undetermined limits, will provide the same benefits; so I
may as well buy a 100w cable or less and put it on a timer for
lots of on-off cycles instead of using a thermostat.  Is this
     My second question is whether, in the interest of not having
to reach down into the tank _quite_ so far, I can get away
with a deeper substrate than is usually recommended, e.g.,
by _TOA_ (let's say 4.5" or 5.5" or higher? as opposed to
3.5"); in other words, if a higher watt cable will mean (A)
that I have less to worry about concerning anaerobic
build-up and therefore can have a deeper substrate or (B)
that a deeper substrate will reduce the effectiveness of the
cable (but _which_ cable?) as far as creating convection
currents.  Maybe there are other ramifications.  I will be using
aquarium gravel 2-4mm and Duplarit-G in the bottom third of
it; I'll also add Malaysian trumpet snails.  Recall that the
2500gal aquarium in _TOA_ cites a substrate depth of 10 to
23cm (p. 175, although the sentence could also be read, in the
English translation, as saying that this measurement is only
for the bottom third laterite layer), with a 340w cable and the
water column at 25C.  Who knows how hot the bottom of the
substrate got, but the aquarium did have a concrete bottom!
The "Substrate table" (p. 185) does _not_ include cable
wattage as a parameter determining substrate depth, and 
implies that the amount of gravel is only a matter of the
aquarium's size (the greatest "Layer height" being 11cm for
a 160cm x 65cm x 50cm aquarium).
     All responses will be appreciated deeply (but _how_

--joey powers
charlestown, ma
joeyp at thecia_net  (please use this address if you email me)