How to raise magnesium in a reef tank

Short answer: dose magnesium chloride — usually blended with a little magnesium sulfate — in an amount calculated for your water volume, up to about 50 ppm per day. Magnesium is the one worth fixing first, because it's what lets your tank hold calcium and alkalinity up at all. Here's how much, and why it matters more than its reputation suggests.

Why magnesium comes first

Magnesium is the quiet gatekeeper. It does two things that make the rest of your chemistry possible: it sits on growing calcium-carbonate crystals and keeps them from taking over, and it ties up carbonate so calcium and alkalinity can stay in solution above their natural saturation point. When magnesium is low, calcium and alkalinity won't hold no matter how much you dose — they just precipitate back out. So if your calcium or alkalinity keep falling and won't stay up, check magnesium before you chase either one.

The upside: magnesium is consumed slowly. You don't dose it daily like alkalinity — test it roughly monthly, and correct it when it drifts.

How much to add

Magnesium takes more product than calcium or alkalinity because there's a lot of it to move (target 1250–1400 ppm). Magnesium chloride is the main ingredient: about 42 g of magnesium chloride (hexahydrate) raises 100 litres (26 US gal) by roughly 50 ppm.

Most reefers blend magnesium chloride with a smaller amount of magnesium sulfate (Epsom salt) — roughly a 5:1 ratio — so the chloride and sulfate stay near their natural seawater balance rather than pushing one too high. Use the exact figure for your tank:

The calculator uses your net water volume and the safe daily limit.

The safe daily limit

You can raise magnesium by up to about 50 ppm per day safely. Because it moves slowly and is rarely urgent, there's usually no reason to rush — spreading a correction over several days is fine and easy on the tank.

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