Cable confusion: what watt?

From: GAMPOW at aol_com
Date: Fri, 19 Apr 1996 13:32:37 -0400

> 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 true?

We have tried lower wattage cables for 2 years now and aren't as happy
with them as with the higher wattage cables, but I'm afraid I don't
have much data to justify that opinion.  The 100g discus tank with
100w cables still had some nuisance algaes that we thought would go
away after the cables were added.  However, the newer 120g tank with
100w cables did not have any algae problems.  Our upcoming
"experiment" will have two 100g tanks with 150w (76 F water temp) and
200w (83 F water temp) cables and a 120g tank with 200-250 watt cables
(76 F water temp).  

Again, one problem with the 250w cables in the 90g tank was that the
cable was "too long" and the runs had to be spaced closer together
than recommended, i.e., if Dupla cable anchors were used as designed.
The 100w cables in the 120g had to be spaced farther apart than
recommended.  I don't know if Dupla established the spacing based on
what "looks good" or if there was science behind it.  The Dupla
anchors do dictate a much wider spacing than is shown in the book (p.
56 and 57).

> 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 ...

I can't see where an inch or two difference in gravel height will make
maintenance much easier.  If I had to guess, I would think that higher
wattage cables might compensate and provide enough circulation.  But 
no one seems to know the true physics behind the cables and convection
currents, so it is all pure guesswork and conjecture. 

Karen Randall has corresponded with Kaspar Horst (Dupla).  Perhaps she
could forward the question to him?