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Reef Tank Magnesium Calculator
Reef Tank Magnesium Calculator — MgCl₂ & MgSO₄ Dosing
Magnesium is the third most important major element in a reef aquarium after calcium and alkalinity. Natural seawater contains approximately 1280–1350 mg/L of magnesium. In reef tanks, magnesium is consumed slowly — primarily through coralline algae growth and small uptake by corals — but evaporation-only top-off (without water changes) can gradually deplete it. Low magnesium causes two serious problems: calcium and alkalinity become unstable (magnesium inhibits the spontaneous precipitation of calcium carbonate, keeping both elements dissolved), and coral coloration and polyp extension suffer. Optimal range is 1280–1380 mg/L. Magnesium can be raised using magnesium chloride (MgCl₂·6H₂O), magnesium sulfate (Epsom salt, MgSO₄·7H₂O), or a combination of both. The mixed approach is preferred because it maintains the natural chloride-to-sulfate ratio of seawater.
How to Use This Calculator
1
Test your magnesium with an ICP test or a dedicated Mg test kit. Note the current value in mg/L.
2
Enter your target — 1280–1350 mg/L mirrors natural seawater. Most tanks target 1300–1380 mg/L.
3
Enter your tank volume in liters. For a mixed reef, estimate actual water volume as ~70–75% of gross tank size.
4
Choose the dosing method. The mixed chloride + sulfate option is best practice — it avoids skewing the ionic balance.
5
Dissolve the powders separately in RODI water. Add over several hours if raising more than 50 mg/L in a single session.
Need reef supplements?
We stock Brightwell Aquatics, Two Little Fishies, and premium additives — shipped with your corals.
Frequently Asked Questions
Why does low magnesium destabilize calcium and alkalinity?
Magnesium competes with calcium for carbonate binding sites. When magnesium is low, calcium carbonate precipitates out of solution more readily, causing both calcium and alkalinity to drop unpredictably. Maintaining Mg at 1280+ mg/L keeps Ca and KH stable and predictable.
How fast does magnesium drop in a reef tank?
Magnesium depletion is slow compared to calcium and alkalinity — typically 5–20 mg/L per week in a mixed reef. It's mainly consumed by coralline algae growth. However, if you top off only with RODI water and do few water changes, levels can slowly drift down over months.
Is Epsom salt safe for reef tanks?
Yes — magnesium sulfate (MgSO₄, sold as Epsom salt) is safe and widely used. The pure, unscented food/pharmaceutical grade is fine. However, using only sulfate raises the sulfate-to-chloride ratio over time; using a mixed approach maintains the proper ionic balance of natural seawater.
What are signs of magnesium deficiency?
Low magnesium (below 1200 mg/L) typically shows as: calcium and alkalinity swings that don't respond to dosing, reduced coralline algae growth, dull coral coloration, and in severe cases, coral bleaching. ICP testing is the most reliable way to confirm low magnesium.
