Sorry for the confusion. DIC stands for Dissolved Inorganic Carbon. It refers to the four forms of carbon in water, CO2/H2CO3/HCO3-/CO3=. The pKa of the H2CO3/HCO3- equilibrium is 6.4, and the pKa of the HCO3-/CO3= equilibrium is 10.4. The pH of water depends on the ratio of these four species with one another. At pH values normally found in aquariums, the bicarbonate (HCO3-) ion is the dominant species of DIC.