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CO2 geeting a bad rap

Much has been said on the list lately concerning too high levels of CO2 in 
the tank.  While too much is 'vague,' I ran across something that describes 
the situation perfectly. I'm not sure if it is the cause, but the symptoms 
fit. It starts out kinda basic, but please bear with me on this. Thanks in 
advance for your indulgence.

'General Information: 

Nitrogen makes up 78% of the atmosphere as gaseous molecular nitrogen, but 
most plants can use it only in the fixed forms of nitrate and ammonium (for 
specific information on ammonium, please refer to Ammonia section). Nitrate 
and nitrite are inorganic ions occurring naturally as part of the nitrogen 
cycle (Smith, 1990). 

Nitrogen Cycle: 

The nitrogen cycle is composed of four processes. Three of the 
processes--fixation, ammonification, and nitrification--convert gaseous 
nitrogen into usable chemical forms. The fourth process, denitrification, 
converts fixed nitrogen back to the unusable gaseous nitrogen state (Smith, 

    Nitrogen fixation is the conversion of nitrogen in its gaseous state to 
ammonia or nitrate. 
    Biological fixation accounts for 90% of the fixed nitrogen in the cycle. 
In biological fixation, molecular nitrogen (N2) is split into two free N 
molecules. The N molecules combine with hydrogen (H) molecules to yield 
ammonia (NH3). 

    The fixation process is accomplished by a series of different 
microorganisms. The symbiotic bacteria Rhizobium is associated with the roots 
of legumes. To a lesser extent, some root-noduled nonleguminous plants also 
exhibit symbiotic relationships with bacteria. Some free-living aerobic 
bacteria, such as Azobacter and Clostridium, freely fix nitrogen in the soil. 
Finally, blue-green algae (cyanobacteria) such as Nostoc and Calothrix can 
fix nitrogen both in the soil and in water, yielding ammonia as the stable 
end product.

    Ammonification is a one-way reaction in which organisms break down amino 
acids and produce ammonia (NH3). 

    Nitrification is the process in which ammonia is oxidized to nitrite and 
nitrate, yielding energy for decomposer organisms. Two groups of 
microorganisms are involved in nitrification. Nitrosomonas(which ones are 
currently being debated) oxidizes ammonia to nitrite and water. Subsequently, 
Nitrobacteria oxidizes the nitrite ions to nitrate.

    Denitrification is the process in which nitrates are reduced to gaseous 
nitrogen. This process is used by facultative anaerobes. These organisms 
flourish in an aerobic environment but are also capable of breaking down 
oxygen-containing compounds (e.g. NO3-) to obtain oxygen in an anoxic 
environment. Examples include fungi and the bacteria Pseudomonas (Smith, 

Numerical Categories: Limits Suggested to Maintain Designated Use: 

                Designated Use                                                
       Limit (mg/l) (AWWA, 1990)
                  Aquatic life - Warmwater fish
                  maximum diversity                                           
     0.1*  (and phosphorus 0.01)
                  moderate diversity                                          
       1.0*  (and phosphorus  0.1) 

Here is what I think is describing what is happening in the tank. Read on:

Health Effects:

    Nitrate concentrations (as NO3-) > 45 mg/l (or > 10 mg/l NO3-N) may cause 
Methemoglobinemia (Blue Baby Syndrome) in warmwater fish (Straub, 1989). The 
toxicity of nitrate in aquatic life is a result of the reduction of nitrate 
(NO3-) to nitrite (NO2-). By reacting with hemoglobin, nitrite forms 
methemoglobin (MHb), a substance that does not bind and transport oxygen to 
tissues. Thus, methemoglobin formation may lead to asphyxia. Normally, 
methemoglobin accounts for 1-2% of the globin in the body. A level greater 
than 3% is defined as methemoglobinemia.  The quick fix for this is rapid 
aeration of the water for 20-30 minutes. In the long run, lowering of the 
nitrate is the only solution.
    Freshwater system impacts: Generally, phosphorus is the limiting nutrient 
in freshwater aquatic systems. That is, if all phosphorous is used, plant 
growth will cease, no matter the amount of nitrogen available. But the higher 
the level of nitrogen does increase the possibility of methemoglobin (MHb).
    The recommended level of nitrogen in freshwater to avoid algal blooms is 
0.1 to 1 mg/l, while the phosphorus concentration is .01 to .1 mg/l. Higher 
concentrations of both will support less diversity (NOAA/EPA, 1988). 

One last point, CO2 has to bind with hemoglobin, other wise it would not be 
transported out the system to allow more O2 in. It does not have as strong of 
a bond as O2 has, but it does bind with the hemoglobin. When you breath in, 
O2 goes to the hemoglobin and CO2 is released from the hemoglobin, so that 
when you exhale, the cycle can start again. Not trying to stir things up, 
just trying to help.

James Watford
Columbia, SC