Aim for magnesium between 1,250 and 1,350 mg/L; natural seawater is about 1,280. Magnesium helps stop calcium and alkalinity dropping out of solution together, so a low reading makes them hard to keep steady. Raise it with a magnesium chloride and sulfate mix, by no more than 100 mg/L a day.
What magnesium does
Magnesium is the third most abundant ion in seawater. In a reef tank, its main job is chemical: it slows the formation of calcium carbonate crystals on surfaces, which would otherwise pull calcium and alkalinity out of the water.
When magnesium runs low, calcium and alkalinity can be hard to keep up however much you dose. Check magnesium first if you’re fighting both.
Testing it
Magnesium tests involve titrations with larger numbers, so small errors matter. A reading of 1,200 may be within your kit’s margin of 1,300. Test twice before acting on a surprise, and every two to four weeks once it’s steady.
If your magnesium never moves between water changes, you may not need to dose it at all. Many soft-coral and mixed tanks hold it with water changes alone.
Raising it
Magnesium supplements are usually a blend of magnesium chloride and magnesium sulfate. Bulk Reef Supply explains the blend keeps the balance of sulfate and chloride in the tank. Its general-adjustment mix is roughly 10 parts chloride to 1 part sulfate by weight.
Bulk Reef Supply also says to keep changes under 100 mg/L a day. By chemistry, about 8.4 g of magnesium chloride hexahydrate raises 100 L by 10 mg/L, so a 50 mg/L rise in a 200 L tank needs roughly 84 g. For a branded product, use its own label dose.
| Raise by | 100 L | 200 L | 300 L |
|---|---|---|---|
| 25 mg/L | 21 g | 42 g | 63 g |
| 50 mg/L | 42 g | 84 g | 125 g |
| 100 mg/L | 84 g | 167 g | 251 g |
Calculated from the molecular weights for the chloride alone. A chloride–sulfate blend needs slightly more by weight.
Keep it in the routine
Test magnesium with calcium every couple of weeks while you’re dosing, and log each one. A slow downward trend tells you to add a small maintenance dose, long before it causes problems with calcium and alkalinity.
Common questions
Is 1,500 magnesium too high?
It’s above the usual range. It isn’t usually urgent; stop dosing and let water changes bring it back.
Can I use Epsom salts?
Epsom salt is magnesium sulfate. Used alone, it adds a lot of sulfate. Most reef keepers use a blend with magnesium chloride.
Does magnesium affect alkalinity tests?
No, but low magnesium can make alkalinity and calcium fall faster through precipitation.
Sources and limits
Last checked 12 October 2026. Ranges and doses here come from the published sources below and from the chemistry of each additive. They’re starting points for your own tank, not guarantees for any coral.
- Randy Holmes-Farley, Reefkeeping: Reef Aquarium Water Parameters
- Bulk Reef Supply: magnesium supplement mixing guidance