PH swings occur daily or otherwise in many tanks as a normal matter, as in any relatively small body of water due to a variety of reasons, without affecting the livestock to any serious degree.
The important key in this regard is the time period over which the swings occur - if they are very quick - i.e. over a short time frame of within 1, up to several hours - that's concerning. If the swings take place gradually over many hours of the day, it shouldn't be problematic.
In my view - as an opinion only - if the swing is say, greater than .4 .5,.6, or more, and happens rapidly, that's a potential problem - if it swings by say, .8 to 1.0 or more over a period of 12 or more hours - that's likely tolerable to the fish, unless it's often repeated, in which case over time this will likely result in damage to health.