For future reference, if you use IO, by following the additive amount of magnesium and calcium that Ben has laid out to each water change, not much else for magnesium would be needed. Mine seldom drops below 1350-1400ppm range.
Unless the tanks using a ton of magnesium that should keep levels up. If you're following another route, then something like the calculator Tom mentioned would show how much needs adding. I have never used Bens that way but many others here have. Its just a version of Randys, {RC} for dosing calcium, magnesium and alkalinity.
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Doug
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