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Carbon dioxide (CO2) is a by-product of decomposition of organic matter, fish and aquatic plant respiration, and can cause problems for most rainbowfishes if levels build higher than 20 mg/L. Most can tolerate concentrations of 10 mg/L provided dissolved oxygen concentrations are high. Well-maintained aquariums normally contain less than 5 mg/L of free carbon dioxide. Although, levels may fluctuate from 0 mg/L in the afternoon to 15 mg/L at daybreak.
Excessively high levels of carbon dioxide (greater than 20 mg/L) may interfere with the oxygen utilisation by the fish and can cause them to lose equilibrium, become disoriented and possibly die. Fish are able to expel carbon dioxide through the gills in response to a difference in carbon dioxide concentration between their blood level and the surrounding water. If environmental carbon dioxide concentrations are high, the fish will have difficulty reducing internal carbon dioxide concentrations, resulting in accumulation in their blood. This accumulation inhibits the ability of haemoglobin, the oxygen-carrying molecule in fish blood, to bind oxygen, and may cause the fish to feel stress similar to suffocation.
Adequate aeration or surface agitation, and buffering of the water will keep carbon dioxide at acceptable levels. Adequate buffering will initially remove free carbon dioxide and store it in reserve as bicarbonate and carbonate buffers. This concept is discussed in further detail under alkalinity. Small water exchanges also reduce the levels of carbon dioxide.
Problems with carbon dioxide are only likely to develop in the aquarium when rainbowfishes are maintained under somewhat crowded conditions or if being added as a fertiliser for aquatic plant growth. In addition, there is some evidence to suggest that the toxicity of carbon dioxide is enhanced by low dissolved oxygen concentrations.
Carbon dioxide can be measured directly with standard test kits. Alternatively, measurement of pH, temperature, and alkalinity can be used to estimate the carbon dioxide concentration in aquarium water (Table). There are important interrelationships between carbon dioxide, pH, temperature, and total alkalinity. However, estimation of carbon dioxide by this method is plagued by difficulty in obtaining accurate measurements.
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Factors for calculating carbon dioxide concentrations in water with known pH, temperature, and alkalinity measurements.
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Temperature (°C) |
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pH |
20 |
25 |
30 |
35 |
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6.0 |
2.112 |
1.970 |
1.882 |
1.839 |
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6.2 |
1.333 |
1.244 |
1.187 |
1.160 |
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6.4 |
0.841 |
0.784 |
0.749 |
0.732 |
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6.6 |
0.531 |
0.495 |
0.473 |
0.462 |
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6.8 |
0.335 |
0.313 |
0.298 |
0.291 |
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7.0 |
0.211 |
0.197 |
0.188 |
0.184 |
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7.2 |
0.133 |
0.124 |
0.119 |
0.116 |
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7.4 |
0.084 |
0.078 |
0.075 |
0.073 |
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7.6 |
0.053 |
0.050 |
0.047 |
0.046 |
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7.8 |
0.034 |
0.031 |
0.030 |
0.029 |
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8.0 |
0.021 |
0.020 |
0.019 |
0.018 |
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8.2 |
0.013 |
0.012 |
0.012 |
0.011 |
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8.4 |
0.008 |
0.008 |
0.008 |
0.007 |
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Factors should be multiplied by total alkalinity (mg/L) to get carbon dioxide (mg/L). For practical purposes, CO2 concentrations are negligible above pH = 8.4. |
© Copyright Adrian R. Tappin
Updated October, 1999.
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