Carbon dioxide levels of below 10mg/l are thought to be well tolerated by fish, although sensitivity to the gas varies between species. The level of CO2 in the water varies with the respiratory and photosynthetic activity of animals and plants in incoming water, the level of decomposition of organic material in that water (a very significant contributor to CO2 levels in some nutrient-rich waters), and the respiration of the fish themselves. CO2 can build up to significantly high levels in systems with large numbers of fish and relatively slow water turnover.The effect of increased CO2 in water is to reduce the rate at which CO2 from the fishs own metabolism can be released from the blood through the gills, thus the CO2 in the blood also increases - this is known as hypercapnia - resulting in a drop in the blood pH, an acidosis. At the same time the oxygen-carrying ability of the haemoglobin in the blood is reduced and the long term this balancing act can have a more profound effect on the health of the fish. Nephrocalcinosis in salmonids has long been recognised as a pathological entity related to high dissolved CO2, eventually leading to the formation of large mineralised deposits within the excretory tissue of the kidney and associated kidney pathology.The condition can result in poor condition and performance and occasional fish loss.