Let me add one that I just thought of: understanding your salt mix.
The reason it's called 'salt mix' and not just 'salt' is that it *is* a mix, of which salt is the main ingredient. [Now and again we have a new hobbyist ask if they can use Morton Salt. The definitive answer is no.]
Fish, inverts, and corals not only breathe water to extract oxygen, they drink it constantly as a source of nutrients which support life. Read the label on your salt mix: you'll find it's a chemist's workshop of other elements, primarily salt (sodium chloride), calcium, magnesium, and a buffer; then an array of such elements as boron, iodine, selenium, and so on down.
The salt mix intended for fish-only tanks costs less: it contains less calcium, for one thing. The salt mix intended for reefs is more complex, and has about the balance of elements you can see listed in my sig line. All creatures with skeletons or shells use calcium---but stony coral and clams use it to build with. They use it so fast you end up having to dose more in, even with regular water changes.
The Big Three items that are in a locked relationship in seawater are: calcium, buffer [which you read as 'alkalinity'], and magnesium. If magnesium is low, the calcium and alkalinity readings will start going down and down, no matter how much calcium and buffer you add. Raise the magnesium first, and your additions of calcium and alk buffer will stay in solution and become available to fish and corals. Coral keeping is just that simple: just keep those elements available, and don't neglect the water changes that supply the little trace elements as well. Selenium is a tiny trace---but without it, corals may 'bail' from their bases. Iodine is another: without it, your crustaceans have trouble in molt.
NEVER, EVER, EVER try to dose trace elements as a beginner: just do your water changes, and these micro-trace elements come in with the salt mix, in perfectly adequate supply. NEVER DOSE ANYTHING YOU DON"T OWN A TEST FOR. When you start to supply calcium, alk buffer, and magnesium for corals, get those 3 tests.
Just as you draw that 'fill line' to help you keep up with salinity, a LOG BOOK, in which you write your test results for these various minerals, will help you see at what rate, say, calcium is declining in your tank. First you think: if calcium or alkalinity is falling, that means my mag is low. Test that. Add sufficient to reach 1300. Then your calcium dose will 'take'. And when you have kept that written record for a while you will find those numbers are regular as a heartbeat: they tell you about the chemistry going on in your water column, how your corals are eating, whether they're eating, and so on. When the numbers change rate---something is going on.
If you're fish-only, this part is simpler. Just do your water changes, 10% a week, forever, and you're good. This is where you have the advantage over reefers, who don't have a filter to change. EVERYBODY gets to do water changes. And the one test I recommend for fish-onlies is alkalinity. You can see by what I just told the stony-reef-folk that alk doesn't exist on its own, and it has to be in balance. But if your fish aren't happy and your ph is weird, alkalinity is a very good thing to check. Likewise: softie coral folk: you're kind of like the fish-only folk, in that you only have to do water changes, but again---if things aren't going well, check your alkalinity. If it's off, take measures as described above.
And if you want lots of pink coralline (acrylic tanks beware, because it gets on your tank walls as well) just raise your magnesium to 1300. You'll see it, if there's any in your tank at all.