Check those links out above for sure!
In a nutshell, there are other factors, but in even a semi-normal situation (good tank with regular water changes) they are barely relevant so they don't get worried about. Alkalinity is king. While the rest of the elements of sea water are important in their way, they just dont need the same level of monitoring or maintenance as alkalinity becuase there is so much of them. For perspective, I think it's said in one of those linked articles that if you could precipitate out 100% of the (bi)carbonates from a batch of sea water, it would barely make a dent in the amount of calcium and especially the amount of magnesium still present in the water.
In fact, the reason "two part" isn't called "three part" is because the Magnesium comes in just over the horizon of being worth worrying about. At most you'll have to dose Mg in the minute quantities specified in both of "Randy's Recipe's"...which is the amount dictated by the composition of coral skeletons. In reality I, for example, have barely had to dose Mg in the ~6 years my reef has been running - off hand I think a dozen times at most. For the vast majority of the time I kept to a strict 20% per week water change schedule using Reef Crystals and that's almost all I had to do for Mg.
Strontium is the next-most-depleted element in sea water after Mg and IME its wa-a-ay less worth worrying about than even Mg. I do not own a Sr test kit.
Moral of the story is that you need to be religious about alkalinity. Calcium is "merely important" by comparison. You only have to keep tabs on magnesium. All else is "for informational purposes". (Of course as with anything there are situational exceptions. For example if you run a calcium reactor or anything else with CO2 injection, pH monitoring will become much more important.)
YMMV significantly if you opt for no water changes or few water changes or use a salt that is not enriched as Reef Crystals is.
Hope this helps!
-Matt
