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Re: substrate CO2 (was Liquid CO2)

On Thu, 23 Aug 2001, Robert H wrote:
> >>James wrote that "compressed gas is the most economical and easiest long
> term solution toadding carbon to an aquarium". How about simply using
> peat in the substrate? cheap and easy! As it decomposes, it will provide
> carbon too. I believe that Diana Walstad has a lot to say about natural
> C sources in her book, "Ecology of the Planted Aquarium<<
> Thats funny because I was just in a discussion about this very thing with
> Tom Barr and MarkieMark in RK aquaria, who both said that plants can only
> utilize this form of CO2 if no other form is available, and will strip the
> KH before using CO2 from sediment.

I think this is wrong.  I've read several references to plant uptake of
CO2 through their roots.  Futher I've reade (from supposedly authoritative
sources, but unfortunately I can't recall the reference) that Isoetes
ferns take CO2 primarily through their roots.

I expect that generally plants that can take CO2 by the roots will take
CO2 through their roots before they get it out of bicarbonate.  Bicarb is
a relatively expensive source of carbon.  Further, switching from CO2 use
to bicarb use isn't either simple or automatic.  I read a detailed study
of two common aquarium plants (E. densa and V. americana) that found that
the switch from CO2 to bicarbonate required certain physiological changes
in the plant.  Further, those changes only occurred when there were
specific environmental triggers.  The authors thought that the trigger was
provided by a high pH at the start of a photoperiod.

All that aside, while organic material in a substrate is a potential CO2
source I doubt that the substrate in most planted aquariums will ever
provide very much.  Even if the substrate does produce some CO2, it does
it at the cost of the oxygen level in the tank.  Producing just 5 mg/l CO2
consumes 3.6 mg/l of O2.  Further, the substrate produces CO2 only under
aerobic conditions.  Oxygen in the substrate is quickly consumed by any
production of CO2 and condition become anoxic.  As a result, there can't
be very much CO2 production from the substrate unless there is active
circulation in the substrate constantly supplying new oxygen.  Plants keep
their roots supplied with oxygen, but that may or may not provide oxygen
to the surrounding substrate.

You can probably get quite a bit more CO2 produced from bacteria working
on organic debris in the filter where there is a constant supply of
aerated water.  The fish and other aerobic critters in the tank also
provide CO2.

The substrate, the filter and other sources are going to produce the 2-5
mg/l background levels that we often see in unplanted or dimly lit tanks.  
For dimly-lit tank that CO2 may be enough.  Certainly the fish and filter
will not produce enough CO2 to supply a heavily planted, brightly lit

As far as the original question over "Liquid CO2" is concerned... I saw
some in a local store several months ago and read the package literature.
I thought it was pretty much the same thing as Seachem's Excel, though the
actual compound used by "Liquid CO2" may be different from what is used
in Excel.

Roger Miller