Magnesium

Magnesium: good for your tank: fyi

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Water balance is a three-legged stool. One leg is calcium, one is alkalinity, and the third is magnesium.
Here's how it works.

Alkalinity should be between 8.3-9.3. You use Alkalinity Buffer (I use Kent) to keep it there. Your water changes also help. [Part of your salt mix IS buffer, which is why you should NEVER use solidified salt: if it's a brick, the buffer's shot.] And you should not add powdered alk and powdered cal at the same time, or you get a snowstorm. 12 hours apart is a fair rule.

Calcium should be between 405 and 420. Never below 405. A lot of the powder in your salt mix is calcium (among other minerals). The ocean naturally dissolves limestones and discarded shells. Unfortunately, to my observation, it doesn't discriminate about living shells, and will dissolve your snails' shells from around them if your water is low in calcium.
What else uses calcium? Fish: bones, muscle, both depend on calcium. Corals use it like crazy: in the presence of the right alkalinity, corals EAT calcium like crazy. People are always wanting to 'feed' their corals: but what corals mostly eat is sunlight and calcium. So whether you're a FOWLR or a reefer, this chemical is important.

Magnesium is the third leg, a mystery of sorts. You should have it at about 1280, 1300. It gets used up by corals and critters very slowly...so you'll be dosing a lot more buffer and calcium than you do mg. (That's the abbreviation for magnesium.) You put the magnesium level at 3x the level you want your calcium to hold. So if you want calcium at 420, you need magnesium at 1280. If magnesium is too low, you can dose alk and cal all day long and their levels will NOT rise. CAN not rise. People are always asking how to get coralline to grow: I assure you, maintain your level at 1280 and you will get a lot of coralline.

So if you're testing, test first for alk. That's quick and easy. Test next for calcium. If they're both fine, often no need to test mg, because if it weren't ok, those levels would not be good.

If they test low, the next thing to do is test your mg level and correct it. Once it's up, you can now add your alk buffer and, 12 hours later, your calcium, and test the next day. You should see improvement. Experienced reefers get a 'feel' for when mg could be running low, and a test in time can let you deliver a small mg dose that will prevent your alk and cal from falling.

Remember that alk and cal get used up much faster than mg, and some salts are low in mg; so watch this 'mystery' chemical and learn your tank's need for it, and you'll have a lot less trouble getting your water to be ocean-like.