Please read through the thread that I link to in my signature area with a link called Hardness shock vs pH strttndrrn. In fact it turns out that fish are not especially sensitive to pH and pH changes but that we often interpret what has happened to a tank as a change in pH rather than looking at what really has happened to the mineral content. Fish adapt quite quickly to changes in pH but we often see pH change in our tanks when other things are happening. It is the mineral content change that harms the fish.
Back to your symptoms, if you have very soft water, which is often the case with neutral to low pH values, you can have the pH move around quite easily with nothing much actually happening to the water. With no pH buffer, a pH swing is harmless but can cause a great deal of angst to a fish keeper.
