From: "Gary" doji at hawaii.rr.com> on 2005.01.05 at 22:17:53(12554)
Title: Aquarium CO2 injection to benefit plant growth and the carbonate system
Dear Ted et al,
Your CO2 thoughts have been in my mind for a long time now, and with the
help of Dr. James Szyper, Aquaculture Specialist, Hawaii/Pacific Sea Grant
Extension Service, I thought I would add my two cents to the discussion.
The addition of CO2 into
the water to accelerate the growth of Aroids such as Cryptocorynes, Anubias,
etc., is valid only to a certain point.
It?s direct effect is minimal at best, injected CO2 can help if the
plants have as much fertilizer, light and proper temperature as they can use,
probably not much otherwise. It?s
also good to remember that during hours of darkness or suboptimal lighting, the
plants reverse the process and use O2 while giving off CO2. That is not
quite correct. The plants do continue to make and/or use both CO2 and O2,
but they do not produce an excess of O2 therefore, it effectively stops,
allowing the CO2 to increase at night. If CO2 were of great benefit, then
plants would grow at the fastest rate at night or in suboptimal light
conditions.
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When CO2 gas is injected
into water it will, at standard atmospheric pressure and temperature, dissolve
into and react with it to make carbonic acid H2CO3 which in turn dissociates into
bicarbonate H(+) + HCO3(-) which dissociates further into H(+) and CO3(--)
the carbonate ion where, if in the presence of a source of
Ca such as in crushed coral, an additional reaction into CaCO3 occurs, a solid
which precipitates out of solution. This is the carbonate
system.
If there is some calcium
carbonate CaCO3 around, such as crushed coral in the aquarium, this will
dissolve any time the pH is below 7, that is, lots of H(+) present. An
additional factor occurs with the release of the extra H(+) atoms in that the pH
of the water will drop becoming more acid, and being more acid, the CaCO3 begins
to dissolve. All of these reactions are reversible, and which way they go
and how much of everything exists tends to an equilibrium that depends on the
pH. At some point, equilibrium will occur and the reactions will only
start again with the addition in this case of either more CO2 or more Ca.
Some plants, Cryptocorynes included, can and will grow better in a slightly acid
solution, but others may do poorly because the acid harms their
chlorophyll. The simple act of water turn over (turbulance) such as that
caused by an active filter moving a stream of water, air stone agitation, or
even the injection of CO2 bubbles, will drive off any excess CO2 from this
equilibrium faster than the standard exchange at the surface of an unsealed
system.
CO2 levels can be raised
in the water, but they may constantly be lost by the above reactions. The simplest and cheapest way of adding
CO2 to an aquarium is to use a container sealed with a secure 2 hole stopper
having a small air pump pushing air via an air line into the bottom of a slurry
of just enough sodium bicarbonate NaHCO3 ? baking soda ? so that it doesn?t all
dissolve, keeping the solution agitated, and having the second hole fitted with
an airline going from the top of the container above the solution transferring
the gas into the aquarium. If the
bicarbonate is well maintained, the gas coming out should always be saturated
with CO2 so the amount going to the plants can be controlled with a valve to
give them what works best. As Ken said, dry ice is, of course, another
albeit costly and complicated way of adding the gas and has
availability,handling and storage problems too not to mention the temperature
considerations. And I wonder if Julius's aquarist friend Bob would have
just as good a growth in his tanks if he shut off the CO2? I think he is
doing everything else very well. Like Ted, I don't know of any commercial
greenhouse experiments, but in culturing algae CO2 is used extensively along
with extremely high light levels for the most rapid
growth.
These processes apply to
CO2 in water, and different results may be found in the air with emergent
plants.
[PC1]
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