Water Quality: Carbon Dioxide
Carbon dioxide is the by-product of respiration of fish, plants and other organisms. The presence of carbon dioxide is not considered a major problem in most aquariums and, in fact, is important for photosynthesis in a planted aquarium. However, high levels can be detrimental because carbon dioxide reduces the ability of a fish's blood to transport oxygen. Fish in water with high carbon dioxide concentrations (>10 12 mg/L for some fish species) can suffocate even if oxygen levels are high. Although highly soluble in water, excess carbon dioxide can be easily removed through aeration if plants are not present.
Well water may contain supersaturated levels of carbon dioxide. To ensure excess carbon dioxide has been removed, well water should be degassed before coming into contact with fish by vigorously aerating the water with an air pump and airstone for approximately 24 hours before the water is added to the tank.
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 photosynthesize. If they cannot photosynthesize, their growth will be slow. They may lose leaves, exhibit poor color and die back. 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.
Because prevention of excess carbon dioxide buildup through aeration is simple, it is generally not measured in most fish-only or lightly planted aquariums. Carbon dioxide can be tested weekly in heavily planted aquariums to ensure that sufficient concentrations are available for photosynthesis.