The sensitivity of corals to ocean acidification depends on the extent to which they can buffer their calcifying fluid aragonite saturation state ((cf)) from declines in seawater pH. While the seasonal response of the coral calcifying fluid (cf) to seawater pH has been studied previously, relatively little is known about (cf) dynamics on shorter (daily) timescales, particularly whether it is sensitive to seawater pH. Here, we use alizarin dye to mark 4days of skeletal growth in the corals Acropora nasuta and Pocillopora damicornis living in situ on Ningaloo Reef in Western Australia. Exploiting newly developed confocal Raman spectroscopy techniques, we imaged the alizarin stains and quantified (cf) between them. We report the first observations of diurnal (cf) cycles, which were found in both species. Our results are consistent with either external control of (cf) by seawater pH or light, or alternatively that (cf) follows an endogenous circadian rhythm.