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[APD] RE: Some thoughts on substrates and some Heat cable debate

Thomas Barr wrote:
> In reviewing some of the discussion that Dupla espouses regarding the
> benefits of cables I am having a difficult time understanding any benefit.
> It would seem based on their logic that adding additional __flow__ of any
> kind would be detrimental, not beneficial.

George & I had a little debate a few years back about flow induction from
substrate cables. I pointed out that if you examine the thermodynamic
properties of water & substrate that it was unlikely that any appreciable
flow is generated by substrate heating cables. The temperature gradients are
just not enough to counter the viscosity effects within the substrate.

To my knowledge, no one has ever measured flow rates from substrate heating
cables. There is a way to do it if someone is interested. See archives.

I expect that the benefits of cables are entirely as you stated, caused by
increased microbial activity from higher temperature.

We know that Fe reduction occurs in a mineral soil substrate with very
little organic matter because we can observe the deposition of iron oxide on
the glass at the interface with higher oxygen levels. Soil with a very low
organic content is all you need such as topsoil prepared using the Krombholz
soil-soup method. In a nutshell, you mix the soil with water to make a runny
pudding and pass it through screens to remove the bulk of the large organic
material. You can also supplement the soil with some peat to increase the
organic content without creating a danger of an extremely low redox. This
will extend the time that the substrate remains productive.

After a period of time, plant roots can so fill a substrate that the redox
potential is no longer low enough to reduce iron. This is because the
substrate depth is relatively shallow and plant roots convey oxygen into the

Refer to the TAG back issue articles by Paul Krombholz for further
information about his observations with Fe reduction & soils in substrate.
I'm sure you can find lots of discussion in the APD archives too.

To my knowledge, no one has directly measured eH (redox potential) in
aquarium substrates although Diana Walstad & I briefly talked about it. It
would require some very specialized equipment that might be available in a
university setting.

Tom, is that equipment available to you?

Steve P

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