All teleost fish calcify and produce structures known as otoliths confined by saccular epithelia in the inner ear. These structures enable detection of acceleration/movement, equilibrium, pressure changes and sound vibrations. In addition, all marine teleosts produce CaCO3 precipitates in the intestinal lumen to serve osmoregulation. The precipitation reaction in the intestinal lumen of marine fish is vital for their survival and the precipitated CaCO3, which they void to the environment, is a substantial contribution to the inorganic carbon cycle. Both calcification processes are enhanced by exposure to elevate environmental CO2, also at levels relevant for short-term climate change, although species differences in sensitivity exists. The present chapter summarizes what we know about the mechanisms of both calcification processes and how elevated CO2 affects them and their related functions. Fish produced CaCO3 is generally Mg-rich and relatively soluble compared to other biogenic CaCO3. Consequently, the fate of voided CaCO3 is relatively uncertain. Although fish produced CaCO3 has been observed in sediments at shallow depths it appears that the majority of the voided CaCO3 dissolves and contributes to elevated titratable alkalinity with depth in ocean waters.