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[Aroid-l] Aquarium CO2 injection and the Carbonate system

  • Subject: [Aroid-l] Aquarium CO2 injection and the Carbonate system
  • From: "Gary" doji@hawaii.rr.com
  • Date: Wed, 5 Jan 2005 12:17:53 -1000

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.  Itís 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.  Itís 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.


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 Ė baking soda Ė so that it doesnít 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.







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