Nutrients can be a little tricky to master at first, but once you get the idea it gets easier.
What you need to remember is that nutrient levels are reactive to many parallel processes that involve them, wether it being consumed by corals, algae, bacteria or by equilibrium processes happening with your rocks and sand.
It is often the case where you dose N/P and it disappears the day after, or significantly dips, and naturally you’ll think that it isn’t enough so you may also increase the dose, but the culprit here is that once you cover the consumption portion, equilibrium steps into place and at certain moment the amount of phosphates bound into your rocks will be enough and from there on it’ll start accumulating and raising in the water column.
For this reason, it is important to keep a close eye on both Nitrates and Phosphates when changing one or the two.
Personally I test every 2 days when I introduce a new change to the system, and keep testing until it stabilizes then move to weekly tests.
This was already addressed by
@Kasrift, and I agree with him.
While Alk fluctuations can indeed cause STN/RTN, they weren’t significant nor fast enough here to be concerned about.
PH isn’t a good indicator here too, he had a single dip, which can be caused due to many other reasons like dosing anything acidic near the test, testing at night or even as plain as a user error. So nothing here to attribute to the RTN/STN neither.
As for higher vs lower Alk - if that was true, corals wouldn’t exist at all, because the Alk at the sea is around 7dKH, not to mention the many reefers that keep their tanks at these and even lower levels for decades.
Personally I keep my alkalinity between 8.0 to 8.5 dKH and been doing so for 16 years on 2 separate SPS dominant tanks and RTN/STN are rare and unusual events with an explanable reason in my experience.
Moreover, and like many other reefers observed- I’d argue that keeping a high Alk is in fact degrading structural growth of the skeleton, resulting in an abnormally thing and brittle branches, require high nutrients to compensate for the higher skeleton growth rate, which in turn result with poor coloring of the coral.