I should probably study PH properties more but this is what I have gleaned from keeping marine tanks:
A target of 8.4 (alkaline) is good but there is always some fluctuation with temperature. Buffering can raise and help stabilize PH due to acid but if you see a lot of drops in a tank with a lot of fish (or an octopus), this could be an indication of not enough oxygen (or, probably more correctly, too much CO2). Most salts will have enough buffer to change your neutral (7.0) RO/DI water to 8.2 - 8.4 and an argonite bottom substrate is somewhat helpful to maintain the target alkalinity.
Tap water can range drastically. In GA, our water if quite acidic (hence the proliferation of azaleas, dogwoods, etc type plants) and mine starts at about 6.4. The RO treatment will neutralize. Neutral water will quickly take on new PH properties when mixed with higher or lower (acidic or alkaline) PH water.