Well if you think about the biological processes in a closed system, pH will usually peak during the day, esp in a tank that has
macro algae but all tanks to an extent. You have corralline alge, regular algae and even corals extracting CO2, either from ambient air or organism respiration, and using it for cell growth, carbonate fixing for calcificationIskeletal growth,etc)
At night with the lights off, the reverse happens, everything is using oxygen from the water column and the CO2 level goes up. The CO2 tends to create carbonic acid( remember the science experiments where you blow into a glass of water thru a straw?) This tends to drive the pH down.
Trying to level out the pH changes is the reason behind the Reverse Photosynthic refugium theory. Lighting an appropriate sized refugium full of macro algae , on an opposite cycle from tank lighting should reduce the tendancy for pH to swing. Interesting that most test kits are not sensitive enough to show a major difference in reading from dakl to light. another reason I recommend a decent meter.
Another thought I would inteject is that some people choose to run their skimmer part time, not wanting to over skim. Thats fine, but for the life of me I don't know why one would shut the skimmer off at night, when the benifits of increased oxy saturation combined with faster outgassing the CO2. I suspect that if one put a pH meter on the tank and shut off the skimmer at night that the swing would be even greater.
Having said all that, a swing in the range you mentioned seems to be fairly normal and of no concern.
BTW this is also why kalk dripping is recomended at night, as the higher pH can help stabilize the system pH. In high demand systems with DSB the possibility exists for carbonate formation to lead to clumping of the sand bed, not to worry but something to watch for , Hope this helps, and maybe someone better at chem can elaborate on it
