When Do You Actually Need to Dose Magnesium?

Why magnesium gets neglected

Most test kit lineups bundle alkalinity and calcium tests for weekly use, but magnesium tests are often sold separately and used far less frequently — many reefers simply never check it unless something else is already going wrong.

Signs you should test magnesium now

• Calcium and alkalinity dosing math seems "right" but readings won't stabilize
• You recently switched salt mix brands (formulations vary meaningfully in magnesium content)
• You've been running a calcium reactor or heavy 2-part dosing for months without ever checking magnesium independently

How to dose it correctly

Magnesium chloride and magnesium sulfate (Epsom salt, food-grade) are the common DIY dosing options, used to nudge magnesium up gradually — large swings should be avoided just like with any other parameter. Commercial magnesium supplements simplify the math if you'd rather not mix your own.

The target and the takeaway

Most reef tanks do well in the 1250–1350 ppm range. Check it monthly alongside your regular alk/calcium routine rather than only when calcium and alkalinity are already misbehaving — by the time those show a problem, magnesium has often been off for weeks.