Ocean acidification typically reduces calcification in tropical marine corals but the mechanism for this process is not understood. We use skeletal boron geochemistry (B/Ca and δ¹¹B) to reconstruct the calcification fluid DIC of corals cultured over both high and low seawater pCO₂ (180, 400 and 750 μatm). We observe strong positive correlations between calcification fluid pH and concentrations of the DIC species potentially implicated in aragonite precipitation (be they CO₃^2- , HCO₃^- or HCO₃^-+CO₃^2- ). Similarly, with the exception of one outlier, the fluid concentrations of precipitating DIC species are strongly positively correlated with coral calcification rate. Corals cultured at high seawater pCO₂ usually have low calcification fluid pH and low concentrations of precipitating DIC, suggesting that a reduction in DIC substrate at the calcification site is responsible for decreased calcification. The outlier coral maintained high pHCF and DICCF at high seawater pCO₂ but exhibited a reduced calcification rate indicating that the coral has a limited energy budget to support proton extrusion from the calcification fluid and meet other calcification demands. We find no evidence that increasing seawater pCO₂ enhances diffusion of CO₂ into the calcification site. Instead the overlying [CO₂] available to diffuse into the calcification site appears broadly comparable between seawater pCO₂ treatments, implying that metabolic activity (respiration and photosynthesis) generates a similar [CO₂] in the vicinity of the calcification site regardless of seawater pCO₂.