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Carbon dioxide is produced by most life forms in the process of respiration. When you (and the fish) “breathe out” you’re releasing Carbon Dioxide. Even plants respire, at night in the absence of light. They liberate carbon dioxide while they grow. Carbon dioxide from the fish and plants will naturally accumulate in water unless the water is adequately circulated. When water circulates and passes into direct contact with the air, the carbon dioxide is expelled in a process called “de-gassing”. Unlike in the air, carbon dioxide and dissolved oxygen can exist in levels independent of each other. For example, it is possible (while improbable) that you could have high carbon dioxide levels and also have high oxygen levels at the same time. When carbon dioxide levels are high in the water, the fish will naturally require that more oxygen to be present as their gills compete to absorb oxygen and expel their carbon dioxide.
Under most circumstances, carbon dioxide accumulations have little real significance to the hobbyist. The important thing to note is this: Carbon dioxide naturally forms carbonic acid in the water and this acidic compound is responsible for pulling the pH in an acid direction. (See pH discussion in water quality section)
SHORT AND SWEET: Carbon dioxide is made through the respiration of plants and animals and drags down the pH unless properly buffered.