Basically, re water---
1. use ro/di
2. dose to maintain 8.3-9.3 alkalinity. Kent dkh Buffer is good---there are others too.
3. dose to maintain 420-430 calcium. Kent turbo Calcium pilled form is good. likewise other choices.
4. dose *if necessary* to maintain 3x the magnesium reading as calcium: ie, if 420 calcium, then 1260 magnesium
DO NOT DOSE ANYTHING YOU dOn"T TesT for....
Now the secret cheat: if you first set the levels by hand dosing, then top off with 2 teaspoons kalk powder per gallon in your autotopoff reservoir, the levels will stay where you set them until the magnesium runs out, without your needing to dose daily. Kalk powder (Mrs. Wages Pickling lime is what I use.) It should be lidded, it should be dispensed by a topoff system, and you need to test weekly to be sure your levels are staying. That's called dripping kalk. If you want to know more about that system, kalk, kalkwasser, or kalkwater is what to look for. It's extremely easy to DIY. A lid on your topoff bucket and a decent topoff system are the heart of it all. It's a lot cheaper than additives. If you are going down the road to stony corals or clams, this is one of the safest, simplest, easiest ways to supplement calcium and alk simultaneously. But be sure to ask questions while setting something like this up. Recommended: a topoff reservoir of at least 5 gallons. With a lid. And a good, number-based, standard scale set of tests, for alk, cal, mg.