Assuming the air in the room is not changing its CO2 content over time, the size of the pH swing depends on the photosynthesis in the tank (more means a bigger swing) and the aeration (perfect aeration means no swing).
Here's my discussion of the pH swing:
The Daily pH Swing
One of the first things that aquarists who measure pH notice is that the pH changes from day to night in coral reef aquaria. This diurnal (daily) change in pH in reef aquaria occurs because of the biological processes of photosynthesis and respiration. Photosynthesis is the process whereby organisms convert carbon dioxide and water into carbohydrate and oxygen. So there is a net consumption of carbon dioxide during the day. This causes many aquaria to become deficient in CO2 during the day, raising their pH.
Likewise, all organisms also carry out the process of respiration, which converts carbohydrates back into energy. In the net sense, it is the opposite of photosynthesis, producing carbon dioxide and reducing pH. This process is happening continuously in reef aquaria, but is most evident at night when photosynthesis is not pushing pH upward.
The net effect of these processes is that pH rises during the day and drops at night in most reef aquaria. This change varies from less than a tenth of a pH unit, to more than 0.5 pH units in typical aquaria. Complete aeration of the aquarium’s water will entirely prevent this diurnal pH swing, by driving out any excess carbon dioxide or absorbing carbon dioxide when deficient (assuming the carbon dioxide levels in the home air are steady). In practice, equilibration of carbon dioxide by aeration is difficult, and this goal is not often attained. Consequently, the pH does change between day and night.
Higher alkalinity implies more bicarbonate and carbonate in the water, and together these serve to buffer the water against pH changes (that is, they resist the change in pH as additional acids or bases are added). So the higher the alkalinity, the lower the diurnal pH swing. Also, the higher the pH, the more effective is the buffering provided by bicarbonate and carbonate in seawater (up to about pH 9), so the higher the average pH, the smaller the diurnal swing. Additional chemicals in the water also help to reduce the pH swing;
borate, for example, buffers against pH changes.
With that all said, however, I do not believe that the actual change in pH each day is particularly important. I won’t go into the reasoning behind this claim here, other than stating that it is my opinion, based on my understanding of how most organisms control their internal pH, but I do not believe that diurnal pH changes that stay within the range of pH 7.8 to 8.5 are particularly stressful to most reef organisms. That is, these changes are no more stressful than being at the same pH all day. A constant pH of 7.9 may be worse for many organisms than a pH that varies from 8.0 to 8.5 each day. Of course, if the diurnal swing takes the pH outside of this range, i.e., below 7.8 or above 8.5, then certain processes take place that should be corrected, as detailed below.
Mainly by the technology IMO.
