What to aim for, and why the number you pick matters less than holding it. Ranges by tank type for each of the three parameters a reef tank consumes.
Open the full calculatorWithin the published ranges, a tank held at a steady 8.0 dKH will outperform one swinging between 7.5 and 9.5 in pursuit of 8.5. Corals adapt to a level; they do not adapt to a moving target. Pick a value inside the range that your dosing can actually hold and defend it.
Alkalinity and calcium are consumed together as corals build skeleton, roughly in proportion, so a tank drinking alkalinity is usually drinking calcium too. Magnesium is consumed far more slowly but does the quiet work of keeping the other two in solution: when magnesium runs low, calcium and alkalinity stop holding at the levels the tests say they should. Check magnesium first when the other two will not stay put.