Heating Pads

> From: Paul Bucciaglia <paul-b at biosci_cbs.umn.edu>

> Substrate heating:  I follow with interest the DIY posts but I do not 
> have enough electrical experience to feel safe about tryng it.  I have 
> heard of an alternative:  heating mats for reptiles placed under the 
> bottm glass (outside)  of the aquarium.

[and later in the digest...]

> From: CrypDude at aol_com

> To be honest, I have abandoned the Dupla policy of in-tank undergravel
> heating because it is cost prohibitive.  I bought an inexpensive and reliable
> heating matt  for use under reptile tanks -- a thin sheet of clear plastic
> with heating elements through it--which I connected through a dimmer to
> control temperature...never have to worry about shorting out, no worries of
> electricuting the fish and you never disturb the coils when you gravel
> vacuum...plus it's about 1/10 as expensive.  You may wish to seriously
> consider this.  It is working wonderfully on my small 20 gallon tank.

Sigh... One MORE TIME.  [And a-one and a-two]

Please be sure to define what you mean by "working wonderfully".  As is
shown in this article (which will be the FAQ on Heating Cables), there 
are several possible "goals" of undergravel heating...

Substrate Heating Cables

contributed by George Booth

Much of the mystery surrounding heating cables is that Dupla has been
careful to hide the rationale to protect their product, i.e., keep it

I think a key concept is that we are NOT trying to mimic what happens in
nature (even though the Dupla description implies that) but we are trying
the achieve an equivalent biological affect. 

In nature, you have sources of underground water moving to the surface or
surface water moving to aquifers due to natural pressure differentials.
Dupla mentions this in terms of "nutrient springs" in tropical streams. In
our aquariums, there are no such natural pressures to cause any movement
(except for UGF, etc). 

The water column will tend to keep the gravel at water temperature through
conductive heating; heat will "seep" downward. However, in glass tanks
especially, the glass bottom is radiating heat into the room, cabinet,
etc, unless insulation is provided. This will tend to keep the roots
cooler than the water temperature. Even with insulation, you'll find the
bottom of the substrate cooler than the top, just not as much. 

Here is a list of substrate processes I think are important (no particular
order of importance implied): 

 1. Provide warmth in the substrate for certain plant species (Barclaya longifolia,
   specifically). In this case the substrate should be warmer than the water. (``hot
 2. Provide warmth in the substrate to speed up biochemical processes. 
 3. Transport nutrients from the water into the substrate. Important nutrients
   would be ammonium (fish waste, etc), iron (from trace element additions),
   calcium, potassium and other trace elements. This will replenish nutrients
   used by the roots and provide long term viability (in terms of years). 
 4. Transport harmful products out of the substrate. Decomposition products may
   be harmful to plant roots. There is also conjecture that plants give off low
   level toxins to keep other plants out of their territory (successful weeds have
   made this an art form). If these toxins build up due to poor circulation, the
   plant may harm itself. 
 5. Provide a chelating medium that binds the divalent state of trace elements
   with an organic molecule, enabling the trace element to be adsorbed by root
 6. Provide a reducing rather than oxidizing environment so that trace elements
   are kept in their divalent state (usable by plants) or are reduced from their
   oxidized trivalent state. Iron especially will rapidly oxidize in water with
   normal levels of oxygen. 

Heating coils provide the ``hot feet'' and warmth for biochemical
processes directly. The convection currents generated by the "spot" heat
source of the coils provide for nutrient and toxin transport. Laterite in
the bottom 1/3 of the substrate provides the chelating medium. The slow
convection currents, coupled with nitrifying bacteria in the gravel will
reduce the concentration of oxygen getting to the bottom layer of the
gravel, providing a reducing environment. 

A heating pad under the tank will tend to warm the entire bottom layer
uniformly. This will provide hot feet and increased biochemical activity,
but I suspect the heat will go through the gravel as conduction and won't
generate convention currents. Thermodynamics theory says that conduction
will occur up to a certain heat threshold and then convection currents
will be formed with more heat. I think the linear hot zones generated by
proper spacing of the coils along with the higher temperatures of the
coils will provide this. Yes, there will be hot and cool zones for the
roots but I think the other factors outweigh this. 

Schemes that use warm water flowing in tubes in the gravel (Bioplast, for
example) won't work, IMHO, because they can't generate enough heat.
Bioplast wraps some tubing around a heater and pipes it through the gravel
with a pump. The first foot or so of the tubing may get hot enough (though
I doubt it) but the water in the coil will cool off rather quickly as it
travels through the tube. If the tube is insulated enough to keep the
water hot, then it won't transfer any heat to the gravel. 

Reverse flow undergravel filtration (RUGF) will provide increased
biochemical activity, toxin transport, and a reducing environment. It may
provide ``hot feet'' if you heat the water before putting it through the
RUGF. Nutrient transport is kind of difficult since the water is usually
filtered before going to the RUGF (to avoid injecting crud into the
gravel) and trace elements probably will be oxidized in the filter
(oxidizing is a bio-filter's purpose). Chelating is a problem because a
RUGF will probably push the laterite up and out of the gravel. Don't get
me wrong, a RUGF may provide the six processes, but it would be difficult
to get it set up with the right flows and even flow across the substrate
and proper mechanical filtering, etc. A coil setup is a "no-brainer" if
you have the correct wattage. 

UGF will provide warmth for biochemical activity, and nutrient and toxin
transport. Hot feet would be very tricky to achieve, if not impossible.
Detritus pulled into the gravel can be chelated by the substrate, but a
reducing environment is almost impossible unless a very slow flow is used
and that would be hard to do evenly across the whole substrate. 

We have three ~100g tanks with coils and one 85g tank with UGF. All grow
plants equally well but the 85g is much more unstable. We think it is
sensitive to too much detritus building up in the gravel; a thorough
vacuuming every 6-9 months perks it up. The coil tanks require no gravel
vacuuming and the 90g tank was rock solid biologically for at least three
years. We replanted at that point because some of the plants had gotten
out of control but we didn't "tear down" the tank - just replanted. 

I think this is the key to the cables - long term stability. Plants will
grow fine without them if you can accomplish most of the six things I
mentioned. Just pulling up plants for trimming every month will accomplish
as lot (stirring up the gravel, moving roots out of their toxin zone,


Hi. Erik again.  Y'know, Neil Frank and I had a dialog on setting up 
an experiment to actually test the main hypothesis ... Do cables keep the
soil aerated any more than heating pads or nothing?  Here's how I'd propose
to perform such a test:

  1. Buy three tanks.  Something simple like 10gals.  Or heck, rubbermaid 
Tubs might even be OK.

  2. In tank #1, put a small heating cable.  Under tank #2, a pad.
Tank #3 is kept without substrate heating (control).

  3.  Fill bottom of tank with potting soil/gravel mixture, and cover 
with small layer of gravel, just like a normal tank.  As mentioned in a 
recent TAG, potting soil will go anaerobic quicker than most other 
substrates, so it's ideal for our test.

  4. Turn on the heaters.  Wait 2-4 weeks.  Don't add fish or plants.

  5. Scoop out a cup of the sludge on the bottom.  Sniff.  Compare.  Voila!

Variations:  change the level of heat per square inch of substrate to find
where it begins to smell. :)  Compare the "dimmer" approach vs hour 
on/hour off cycles... In theory, the long on/long off approach should be 
more effective because it applies full heat which moves the water (when 
on), while the dimmer approach may not apply enough heat to convect the 

Anyone got several small tanks they want to try this on? :)

Erik D. Olson                		E-mail-o-meter:
olson at phys_washington.edu             	it's back up!