I'm going to start very slow, very basic, and work toward the lesser known things.
1. your salt mix is not just salt. It's dry seawater. And that means not only salt, it's calcium, magnesium, iodine, selenium, boron---read the label on your salt mix.
2. evaporation never evaporates the minerals. It only evaporates the water.
3. animals and plants suck up part of the minerals. So TWO actions unbalance your tank---evaporation increases the mineral concentration---and animals and plants take the minerals they need and leave what they don't. This means certain minerals run lower and lower and certain ones don't---they just pile up. As you add more fresh ro/di (water that's ONLY hydrogen and oxygen, with NO minerals) your tank gets no more minerals. And your shortage of what the animals and plants are using most gets bigger and bigger. Water changes, 20% a month, replenish the missing minerals, but they're alway playing catchup.
4. Salt mixes are NOT all the same. Reef salt contains a high amount of what corals need AND what fish need. Marine salt for fish-onlies is lower in calcium and other minerals. That's why the price difference.
5. Now---the nitty gritty of chemical balance in your tank. First, the balance is set by your salt brand. The more you mess with that, the worse your water. Translation: don't go dumping supplements into your tank unless you've got the corresponding test. They don't sell these things together because they don't run out at the same rate, but your lfs should stress, with every supplement---you need a test. And you need a logbook. If you turn up a shortage, you dose until you put the RIGHT amount into your tank, with a little leeway; and you test again next week to figure out how fast that's running low. And you dose to stay in the 'good zone', NOT as make-up after your water's gone wonky. Dose to the TREND of the numbers, the way when you're balancing something in your hands, you don't let it swing way to one side before you correct it back to center. There is NO one answer to these things. Every tank is different. And staying in the center of a 'good numbers' zone is best: that gives you a little leeway in either direction.
6. THREE readings go in 'lock' to keep your water good. These three are: the alkalinity of your water, the amount of calcium in your water, and the amount of magnesium in your water. Those of you with freshwater experience are used to tracking PH. Alkalinity is the thing most reefers track. Get it between 8.3 and 9.3 on the KH scale, and don't angst over the ph.
The second reading is your Calcium level. It should be between 420 and 500. Below that---your snails' shells start dissolving. And your fish's bone and muscle suffer. The third reading is Magnesium. All you people who want coralline to grow---just keep this one at 1300. But it does a lot more than supply coralline. It LOCKS the other two readings in a 3-way balance. Keeping everything in that relationship will make everything happy.
7. Remember that business about plants and animals using up minerals? Calcium and magnesium are the ones animals use bigtime. Plants---use phosphate and nitrate. Yes, even those chemicals are useful. Plants grow like mad with phosphate. Grow them in your sump, divide the mass in half periodically and get rid of it, and you've just tossed a lot of phosphate and nitrate. That's what a fuge does. And the reason not to use conditioned tapwater? City water grows plants. Algae. A lot of it. The conditioners don't remove phosphate.
8. Dosing: you must dose to keep your calcium supply up if you have stony coral OR clams. Hand-dosing is just fine if you don't. You should be able to keep up with the mineral consumption problem if you have fish and softies, including anemones. Just stay in the target range, and do your water changes.
If you have, or want to have, stony coral, you need to get onto that calcium situation the minute you put them in the tank. They come in 'asleep'. Given good lighting (a requirement for stony coral) and correct chemistry---they'll put out a finger to feel the water. And they'll start waking up. Hungry---because they've not eaten in a while. And what they want is calcium. A lot of it. They'll suck it right out of your salt mix, until your snail shells start dissolving. So you have to put it in. 3 little coral frags can take heaping teaspoons worth of calcium supplement---daily---and at nearly twenty dollars a jar, this could get ruinously expensive. But there ARE cheap ways to give them what they need. Kalk drips are the cheapest. They can fully supply a 50-60 gallon packed reef. Above that you get into calcium reactors, which can supply much larger reefs. There is also the Balling method. And the 2-Part. Tank size and coral load will determine what you need.
9. aging tank: reading all this should tell you that the older a tank gets, the more little imbalances and shortages it accumulates. Age has benefits, but it also has problems. I recommend, at least every couple of years, an aggressive program of semi-weekly 20% water changes, so you can sort of re-set the balance. It's my own notion, but I think it does a bit to replenish the things far down the list of reef-salt ingredients.
