How long does it take to raise magnesium?

At the safe limit of 100 ppm per day, a 150 ppm correction takes two days. Large volumes of solution are normal for magnesium.

Does Epsom salt alone raise magnesium safely?

Short term yes, but Epsom alone skews sulfate high over time. Use a chloride and sulfate blend for repeated dosing.

ReefMath

How to Raise Magnesium in a Reef Tank

Magnesium corrections are big and slow: raising a 100 gallon tank by 100 ppm takes the better part of a liter of solution. Cap the raise at 100 ppm per day and let the correction take the days it takes.

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Steps

Use a magnesium chloride + magnesium sulfate blend (BRS mix or Randy recipe 3A) so sulfate stays in natural proportion.
Calculate the total dose below; the calculator splits multi-day corrections automatically.
Dose each day into high flow, retest the next day before continuing.