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RE: UndergraverHeater -- Getting to the bottom

> Date: Mon, 9 Dec 2002 23:51:40 -0600
> From: "Charles Kuehnl" <ckuehnl at cox_net>
> Dupla...
> promotes heating cables as a way to maintain an aquarium
> in a manner that is suitable for good growth of all plants for an
> extended period of time.  In the book I believe they mention that
> one of their first test aquariums (10,000 liters) for their method
> developed an irreversible problem that required it to be dismantled
> after 11 years.  They used heating cables and the longevity
> problem went away.

My interpretation of the longevity problem was that they used fine
gravel (like beach sand) for the substrate and detritus eventually
"cemented" the fine grains together. When they went to coarser gravel
(2-3 mm), the problem went away. IMHO, I don't think the heating
cables played much of a role in this particular problem.

> They also mention using a very low wattage cable so that they
> are on most of the time to provide a more constant and gentle
> flow ("as slow as possible").

Keep in mind that "very low wattage" is relative. We use a 250 watt cable in
our 120g, glass, open top tank runnning at 76F (probably too high but we
didn't know any better when we bought our first set of cables); a 200 watt
cable in our 100g, acrylic, hooded discus tank running at 83 F; and a 150
watt cable in our 100g, acrylic, hooded gourami tank running at 80 F.

Also, I don't think the cables should be on most of the time. IMHO, the
do their job of moving water by creating convection currents. To create a
convection current, you need a heat differential. Cool areas and warm areas.
If the cables are on all the time, the substrate will heat up pretty
uniformly with
a small top to bottom gradient. I would suspect that this situation would
promote conductive heating, without much water movement.

If the cables cycle on and off "appropriately", there will be horizontal
cool and
warm zones; warm near the cable, cool between cables. I can picture this
promoting better water movement. Pure conjecture, of course. A real
would be tedius and time consuming.

> Scott continued:
> "Its occasional use seemed to push up into the water column stuff from
> the gravel that "collected" during the off periods".
> Isn't that what it is supposed to do?  I thought that the idea was to
> get that stuff (Dupla calls "decayed matter") up into the column where
> it could be dealt with by the wet/dry filter.

My view is that the convection currents move nutrients *from* the water
column (fish waste, Dupla daily drops, etc) into the substrate where
the CEC of the laterite can bind (sort of like chelating) the nutrients
until plant roots can adsorb them. Metabolic waste products,
alleochemicals, etc are moved out of the substrate to keep the plants
from harmng htemselves or neighbors.

> The OA book says
> "One of the possible reasons for the natural end of the first fresh
> water period (my note: it was originally a salt tank), of the aquarium
> after 11 years, was that the substrate had become too dense over the
> course of time, and had developed a substrate layer which could no
> longer be penetrated by the plant roots.  Decayed material (food and
> plant remnants) settled between the rougher gravel and was mineralized
> by bacteria.  Finally this had the same effect as natural cement, and
> baked the substrate layer together as if it were concrete."

I thought it was "finer sand", not coarser gravel. Coarser 2-3mm gravel
cured the problem. I might be remembering that wrong...

> OA also mentions the use of 2-3mm gravel in conjunction with
> the heating cables as a way of preventing the stuff from getting
> down there in the first place.  Without the cables being on most
> of the time I do not see how the settling of decaying matter would
> be prevented.

The coarser grain keeps it from compacting too much. IMHO.

> Though it might not be the best method, perhaps some indication of the
> validity of the heater cable method could be determined by
> making these measurements on several tanks (with and without heater
> cables) that have been in operation for extended periods and compare the
> results with some allowance given for differing substrates.

We did side by side comparisons of cable and no-cable tanks.  I think
they were valid comparisons even though it wasn't a well controlled
experiment. Reread my article on my website or in your old issue of
Planted Aquarium magazine (same article).

George Booth in Ft. Collins, CO (gbooth at frii dot com)
 The website for Aquatic Gardeners by Aquatic Gardeners
   http://aquaticconcepts.thekrib.com/  (mirror)