Ammonia and nitrite are what kill fish. Unless you are keeping super sensitive species of fish, a pH that is slightly higher or lower than ideal is not going to kill them.
However, a constantly fluctuating pH is going to cause stress. As long as your pH is stable that's generally all that matters. Don't use any products that are designed to temporarily adjust pH as these are going to make matters worse.
Do you test for anything else such as ammonia? You have to understand that there is more to do with maintaining correct parameters than just testing for pH.
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