The pH does impact fish and rapid changes are debilitating. A change of 0.3 over a few hours is nothing, as this occurs in nature every day. Planted tanks usually have a pH fluctuation daily of around this, lowest in the early morning and highest in the evening. This is due to the CO2 which builds overnight during darkness so it acidifies the water and thus the pH will be its lowest in the very early morning, then as the plants photosynthesize the CO2 is depleted during daylight and pH rises.
Fish can withstand a pH shift of close to 1 full degree, but I would certainly not suggest this is OK as it is still debilitating. The fish must adjust its blood pH to match the water, and this takes energy and saps it from other more vital processes, weakening the fish.
As you mention planted tanks with CO2 diffusion...there is thinking now that this does harm fish. The CO2 is not at all "normal" or "natural," and there are many planted tank aquarists who have tanks of plants with no fish. And that is probably a very good approach. I will never use added CO2 in any form in my tanks as the fish are my focus and their health is more important than growing this or that plant. I have fairly heavily planted tanks (see photos below) but they are natural, no additives other than minimal liquid plant fertilizer once a week and with moderate if not low lighting, so the plants that accept this thrive and those that don't get tossed.
