The fluctuation is completely normal and also exists in nature due to variation in CO2(aq) values, which in turn is due to photosynthetic activity. No animal will take harm and there is no benefits in trying to reduce these variations too much. Second: You can never never reduce these variations with dosing carbonates as this alterations is not due to loss of alkalinity, but as I said, natural fluctuations in CO2(aq).
In nature these variations are a little smaller as the ocean has the advantage to "breath" against air with 400 ppm CO2. Inside we almost have values in the air above this 400 ppm of course, so we alway have some lower pH overall. As a general rule you can state that if the air inside has a CO2 of 800pm(common value of inside air with reasonable ventilation situation) you will lower the pH in your aquarium all over the cycle with around 0.3 pH. The ocean fluctuates between maybe 8.2 to 8.4 with a dKH of 7. In an aquarium situation you will with same dKH have a pH between 7.9-8.2. It is important to say that you cannot rise the pH som much with rising only dKH as the relationship between dKH and pH is not linear, so even if you raise you dKH to for instance so high as dKH10, you will not affect the pH so much (maybe 0.05-0.1 as maximum) . So never try to raise pH with raising dKH as long as dKH is above 7. Go to the source of error, that is, increase the possibility for the water to get rid of CO2...and that is achieved only in one way: Take air to the skimmer from outside or even better, let the air to the skimmer go through a canister with calcium hydroxide(Divesorb), so called CO2-sceubber.
Your values indicates that you have not a pH issue at all, as even your lowest value is quite high. I should do nothing in your place, if you are sure that your pH -probe measure correct.
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