Yes, that pH change is very typical.
I discuss it in several articles, including here:
https://www.reef2reef.com/forums/re...-coral-reef-aquarium-randy-holmes-farley.html
from it:
pH
pH is a measure of the concentration of protons (H+ ions) and hydroxide (OH-) ions in the water. Aquarists spend a considerable amount of time and effort worrying about, and attempting to solve, apparent problems with the pH of their aquaria. Some of this effort is justified, as true pH problems can lead to poor animal health. In many cases, however, the only problem is with the pH measurement or its interpretation. Moreover, the maintenance of appropriate alkalinity in seawater goes a long way to ensuring that the pH is acceptable, with just a couple of exceptions that will be discussed below.
Several factors make monitoring a marine aquarium's pH level useful. One is that aquatic organisms thrive only in a particular pH range, which varies from organism to organism. It is therefore difficult to justify a claim that a particular pH range is "optimal" in an aquarium housing many species. Even natural seawater's pH (8.0 to 8.3) may be suboptimal for some of its creatures, but it was recognized more than eighty years ago that pH levels different from natural seawater (down to 7.3, for example) are stressful to fish. Additional information now exists about optimal pH ranges for many organisms, but the data are inadequate to allow aquarists to optimize pH for most organisms which interest them.
Additionally, pH's effect on organisms can be direct, or indirect. The toxicity of metals such as copper and nickel to some aquarium organisms, such as mysids and amphipods, is known to vary with pH. Consequently the acceptable pH range of one aquarium may differ from another aquarium, even if they contain the same organisms, but have different concentrations of metals.
Changes in pH nevertheless do substantially impact some fundamental processes taking place in many marine organisms. One of these fundamental processes is calcification, or deposition of calcium carbonate skeletons, which is known to depend on pH, usually dropping as pH falls. At a low enough pH (somewhere below pH 7.7) coral skeletons can begin to slowly dissolve. Using this type of information, along with the integrated experience of many hobbyists, we can develop some guidelines about what is an acceptable pH range for reef aquaria, and what values push the limits.
The acceptable pH range for reef aquaria is an opinion rather than a clear fact, and will certainly vary with the opinion's provider. This range may also be quite different from the "optimal" range. Justifying what is optimal, however, is much more problematic than is justifying that which is simply acceptable, so we will focus on the latter. As a goal, I'd suggest that the pH of natural seawater, about 8.2, is appropriate, but coral reef aquaria can clearly succeed in a wider range of pH values. In my opinion, the pH range from 7.8 to 8.5 is an acceptable range for reef aquaria.
In truth, many aquarists never measure pH, and many that do so do not do anything with the results they obtain. This lack of action is usually okay, as most aquaria do not naturally fall outside of the acceptable ranges. Times when it is most important to at least check pH once in a while are:
1. When using very high pH additives, such as limewater (kalkwasser). In this case, one should ensure that the pH does not get above about 8.55. At higher values, the precipitation of calcium carbonate on pumps and such can become excessive. Every 0.3 pH unit rise in pH is equivalent to about a doubling of the calcium or alkalinity value in terms of the likelihood of precipitation of calcium carbonate (because bicarbonate turns into carbonate as the pH rises, driving precipitation). Aquaria may often get to a pH that is high enough to double the precipitation rate due to elevated pH, but one does not often see aquaria with calcium or alkalinity that is double the normal value, making high pH a big driver of precipitation.
2. When the air around the aquarium has elevated carbon dioxide levels, such as in a newer, tighter home. Low pH due to elevated carbon dioxide in the air is VERY common. While it may be useful to ensure the pH stays above 8.0, there are many fine aquaria with the bottom end of the pH range at pH 7.8. Below that value, I'd want to take more aggressive action, such as more fresh air in the home, top off with limewater (kalkwasser), a fresh air line from outside to a skimmer inlet, or a CO2 scrubber on a skimmer inlet.
and
The “How To†Guide to Reef Aquarium Chemistry for Beginners Part 3: pH by Randy Holmes-Farley - Reefkeeping.com
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. 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.
