Magnesium is the parameter people test last and should test first. It is what keeps calcium and carbonate in solution at reef concentrations, and when it runs low they precipitate out about as fast as you can dose them. Weeks get lost to that.
Worked from your tank's real water volume rather than the number on the box. The calculator itself needs JavaScript; everything it assumes is written out below.
The symptom is numbers that will not hold. You dose alkalinity, it reads low again the next day; you dose calcium, same. Low magnesium is one cause of that, not the only one, but it is the cheapest to rule out.
Magnesium is used far more slowly than calcium or alkalinity, so it is easy to forget it exists until a water change stops covering the gap. A tank that has not had one in a while is the tank to test.
The two common salts are not interchangeable in what else they add: chloride and sulphate. Dosing a large correction entirely as one of them moves more than magnesium, which is another reason to spread it out.
1250 to 1350 ppm, near natural seawater at about 1290 ppm. The calculator accepts 1150 to 1500 ppm. Magnesium is the most abundant of the three and the slowest to change, which is why a tank can sit low on it for months without anyone noticing until alkalinity stops holding.
Because low magnesium lets calcium and carbonate precipitate out of the water almost as fast as they are added. Magnesium ions sit on the growing surfaces of calcium carbonate crystals and slow them down; with too little of it, a tank can be dosed for weeks and never hold its numbers. If alkalinity or calcium will not stay up no matter what you add, this is the reading to take first.
100 ppm a day, and the calculator splits anything larger. Raising magnesium by several hundred ppm is a week-long job rather than an evening's, and per ppm it takes 0.84 g of magnesium chloride hexahydrate per 100 litres or 1.01 g of magnesium sulfate heptahydrate. Most reef mixes blend the two so that neither chloride nor sulfate builds up faster than water changes remove it.
The rules that apply to every dose rather than to this one, including never dosing calcium and alkalinity together, are on the <a href="/tools">calculators hub</a>.