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Re: Undergravel Heater -- Getting to the bottom

Scott wrote:
"Although OA talked about "low heat" cables, their's were a
whole different ballpark from, e.g., Dennerle's.  I think
George (and I risk misinterpreting his comments again) has
suggested that the cable heat should be high enough to
generate temperature differentials in the substrate.  Very
low wattage cables might not do a good job of that even
thought the low(er) wattage might allow them to run longer.
 Anyway, even very low wattage cables would not help me in
the summertime since I don't need to add *any* heat to the
tanks.  In fact I use AC and a room fan, in addition to the
hood fans, in the summer to keep the tanks from getting too

I have used two types of under gravel heaters. The first was a homemade
epoxy covered copper manifold which I described in a post several years ago.
My current system uses AZOO cables controlled by an Aqua Logic Digital
Temperature Controller (both obtained from Monolith Marine Monsters). Both
systems produced a measurable temperature differential within the substrate
(higher over the cable or heat source, lower in the space between cables or
tubes). The temperature differential was measured using a high resolution
mercury thermometer originally bought for use in color darkroom work - it
reads to 0.1 C.

The question regarding heat differentials within the substrate isn't IF they
are produced, its how BIG a differential is required. In both of my systems
I could see a 3' C gradient - it this enough or too little? I certainly
don't know.

Unlike George, I don't have air conditioning and the tank is lit by 6 - 55W
compact fluorescents (remote ballasts) in an enclosed hood. During the
summer months, the substrate hearing never switches on and even during the
cooler months the heat coming from the lights keeps them off for most of the
afternoon and evening. I think that it is highly likely that the maximum
benefit from cables would be obtained by someone who lives in a northern
climate (i.e. like Germany, or Holland, or Canada) and who doesn't have
central heating. There is a lot of talk in older literature about protecting
the substrate of an aquarium from getting cold feet. Something like that can
only be appreciated by someone who grew up in a house that was almost icy
first thing in the morning when your primary concern was to get a fire going
in the stove to take the chill out of the air. In a case like that, the
cables would probably go a long way to helping tropical aquatic plants grow.
In a centrally (sensibly) heated house or apartment, I think the worry over
"cold feet" in an aquarium substrate is probably a thing of the past for
most of us.

Charles Kuehnl wrote:
"Anyway, it was suggested at that time by one and
seconded by several others here on APD to pick one method you thought
you wanted to try and follow it, listening primarily to the main
proponents of that method and kind of ignore what others would suggest.
It was said that to take a little of one method, then add some advice
from a follower of another method and perhaps add a little from yet some
other method was to potentially court disaster."

Sensible advice, sounds like something I might have told you <g>. Like you,
I picked George as my "expert" during the period I was learning the "high
tech" approach to plants.

Regarding Charles' questions about detritus in the substrate getting forced
into the water column, I never vacuum my substrate. I do use a power head,
in conjunction with high power mechanical filtration to sweep up detritus on
the surface of the substrate every now and again (when it builds up past my
tolerance level). But I have a rather high tolerance level for detritus on
and in the substrate - I prefer not to get too prissy about it. So in my
tank, detritus DOES build up in the substrate - but it only enters the water
column (in a noticeable manner) when I replant a section. And whole sections
DO have to be replanted every now and again, primarily because the plants
grow so thickly that I fear the substrate will become blocked. In this tank,
C. crispatula var. balansae grows like a weed - it now "owns" most of the
central rear section of the tank and I have to constantly root prune to
prevent it overtaking everything else. The central section of the stand is
long overdue for a complete overhaul as I can't get a finger into the gravel
due to the mass of roots. Conversely, C. pontederiifolia, which grew
spectacularly for me in another tank, refuses to do anything other than melt
in this tank. It reached the point where I had to remove all of the C.
pontederiifolia in order to save it from total collapse. During the
transplanting, quite a bit of detritus entered the water column, but it was
removed by the mechanical filters quickly.

"Maybe James Purchase's idea to do some
quantitative measurements would be a good idea although that sounds like
a lot of work for a long time for perhaps a little bit of information
that may only be useful to a few"

It would certainly not be something which an average hobbyist could easily
study - the ion probes I was referring to are expensive and sensitive
instruments and most likely only a university/government/industrial lab
would have access to such equipment. As for the usefulness of the
information, I think it would extend to more than a few - if we actually
KNEW what was going on within a substrate, it would certainly cut down on
the conjecture that we rely on today (and to which this whole thread is

Like Scott, I cannot address George's contention that cables offer long term
stability unavailable to non-cabled tanks. I only have one really big tank
and it hasn't been running unchanged for 10 years. Given the quality of
advice I have received from George over the years, I'd tend to give a lot of
weight to anything he has to say, so he's probably right.

James Purchase