Setting your water parameters to stay....

[Sk8r]

First, get test kits that give you numbers, not colors. Life will be easier.
Salifert tests are numbers with few exceptions.

As you set up for fish or corals, your very first fill should be ro/di water: why? because minerals in water don't leave with evaporation. With successive topoff, they just pile up and up.

You WILL want an autotopoff. A 50 gallon tank can evaporate a gallon a day, easy.

Assume you've got all that, and a decent salt. [Read the label on your salt: it contains every trace you'll need, so don't buy trace elements. If you don't think your salt mix is doing you right, change brands, but you can have bad outcomes with trace elements if you start supplementing and don't know what you're doing.] Also, don't use salt that's gone hard. Likely the buffer that's in it is shot (has already combined with moisture from the air and set like concrete. Keep your salt bucket tightly lidded, and doublewrap and seal any bags of salt.

So, now, you want to test your water.
You need 4 tests. 1. Salinity: get a salinity tester (battery powered) ---They're cheap. I use one from 'Farmers Supply' or the like.
2. dkh alkalinity. 3. calcium 4. magnesium. I use Salifert tests, and Kent supplements to make the changes the tests indicate.

NORMALLY the test you run first is Alkalinity, because it's the easiest test to run and it's an indicator whether the other two tests need to be run at all.

BUT the first time out of the gate, run the Magnesium test first. Why? Because normally magnesium is very stable, doesn't deplete fast, and you don't have too much fuss with it, but IF it's off, ain't nuthin' happy. First check that. It should be at about 1300-1350. THat's a middle-range reading. If it ever drops below 1200, nothing you can do with alkalinity buffer or calcium will do any good. So it is a starting point for a new tank, and a thing to check if your other readings are weird.

When you add supplement of any sort, wait 24 hours (at least 8, but who's getting up at 3 am to add buffer?) before re-testing, because it takes time for what you added to work its chemistry. THAT'S IMPORTANT.

If you need more, add more.

Second test to run is alkalinity. Your reading should be about 8.3. Again, middle of the road. Below 7 is Not Good. To correct it, add dkh buffer.

Third test is calcium---420 is a good reading, again, middle of the road.

Now that you're perfect, you could have either fish or corals or both. If you want STONY coral, clams, etc, you need to watch that calcium level, because they use a lot of it. You can supplement it by putting kalk in your topoff water, at the rate of 2 tsp per gallon. I just dump a packet of Mrs Wages Pickling Lime (I am serious, here) into my 30 gallon topoff reservoir and forget it for a while. Except for keeping that reservoir full. You don't even have to measure, because ONLY 2 tsp per gallon WILL dissolve. The rest just lies there waiting for more ro/di gallons to arrive. [When I say a LOT of it, stony coral once it takes off can use up a jar of Kent supplement in a week, and that would get real xpen-sive. Mrs. Wages is cheap.

TO MAINTAIN PERFECTION:
check your alk once a week. If it starts to sink, that means your magnesium has finally run down. Test and re-set that, then check your calcium level, and alkalinity, waiting the requisite hours between each supplement dose. If you have kalk in your topoff, it will be self regulating so long as that magnesium stays up. This is how the owner of an active reef can safely go on a 3 week vacation to the Bahamas and trust their tank maintenance to a complete novice: you leave them enough barrels of ro/di water and tell them 'Just keep this barrel full.' Set up an autofeeder (sure, chopped clam is great, but your fish will be just fine on flake or pellet for 3 weeks, and fish poo feeds the corals and snails.)

Hope this helps. It's an old-fashioned method that works without dosers, and when you're just starting and don't have spendy systems, it helps maintain your tank, be it reef or fish-only, rock-steady in chemistry, and teaches you a lot about tank chemistry in the process.