Carbon Dioxide
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. Wellmaintained aquariums normally contain less than 5 mg/L of free carbon dioxide. Although, levels may fluctuate daily from 0 to 15 mg/L.
The presence of carbon dioxide is not usually a major problem in well maintained aquariums. However, high concentrations can lower the pH of the water and limit the capacity of fish’s blood to carry oxygen. Fish are able to rid themselves of carbon dioxide through the gills in response to a difference in carbon dioxide concentration between fish blood 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 the blood. This accumulation inhibits the ability of haemoglobin, the oxygen carrying molecule in fish blood, to bind oxygen, and may cause them to lose equilibrium, become disoriented and possibly die even if oxygen levels are high. There is some evidence to suggest that the toxicity of carbon dioxide is enhanced by low dissolved oxygen concentrations.
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. Carbon dioxide can build up to significantly high levels in systems with large numbers of fish and relatively slow water turnover.
Advanced aquarists may use carbon dioxide injection systems to ensure that sufficient levels are available for the plants. Some injection systems will monitor pH levels and inject carbon dioxide as pH levels rise.
In some heavily planted aquariums, carbon dioxide can be a limiting factor. If the respiration of the organisms in the aquarium cannot produce enough carbon dioxide, plants will not be able to photosynthesise. If they cannot photosynthesise, their growth will be slow; they may lose leaves, exhibit poor colour and may even die. Carbon dioxide should be monitored in heavily planted aquariums to ensure that sufficient concentrations are available for photosynthesis. Carbon dioxide can be measured directly with a relatively easy titration test kit, which is fairly accurate.
While oxygen and ammonia levels are often viewed as critical to fish health, carbon dioxide tends to be ignored and very few aquarists regularly monitor carbon dioxide levels in most fish-only or lightly planted aquariums. This is partly due to an assumption that, if the other water quality parameters, particularly oxygen, are OK, then carbon dioxide will not be an issue. Research would suggest that poor water quality in aquariums, particularly high carbon dioxide, may lead to an increased susceptibility of fish to pathogens which later leads to disease.
By paying appropriate attention to this aspect of water quality in the early stages, this may reduce vulnerability to disease. It has also been suggested that high carbon dioxide levels may cause bone demineralisation resulting in spinal abnormalities.
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. Small water exchanges will also reduce the levels of carbon dioxide.