Magnesium (Mg) doesn't get the attention alkalinity does, but it's what makes stable calcium and alkalinity possible. Natural seawater has about 1,285 ppm; Holmes-Farley suggests 1,250–1,350 ppm, with 1,200–1,400 acceptable (Randy Holmes-Farley, "Do-It-Yourself Magnesium Supplements for the Reef Aquarium," Reefkeeping Magazine).
What magnesium actually does
Seawater is supersaturated with calcium carbonate, so it would precipitate on its own if nothing stopped it. Magnesium ions bind to the surface of growing calcium carbonate crystals and "poison" them, stopping the growth. When Mg is low, calcium and alk precipitate on heaters, pumps and sand, and they become hard to hold at target (Randy Holmes-Farley, "A Simplified Guide to the Relationship Between Calcium, Alkalinity, Magnesium and pH," Reefkeeping Magazine).
Signs of low magnesium
- You keep dosing alk and calcium but they won't rise or stay up
- White crust on heaters and pump impellers
- Coralline algae slows or bleaches
- Test reads below ~1,200 ppm
Why magnesium drops
Corals and coralline algae incorporate a little Mg into their skeletons. Kalkwasser and many calcium reactor media contain little Mg, so tanks relying on them can drift down over months. Mg usually drops slowly (often well under 1 ppm per day). If your Mg test swings 50–100 ppm day to day, suspect the test, not the tank. Validate against new saltwater mixed to 35 ppt or an ICP test.
How to raise magnesium
| Product | Per 1 ppm Mg per liter | Notes |
|---|---|---|
| Magnesium chloride hexahydrate (MgCl₂·6H₂O) | 8.37 mg | Adds chloride; fine alone for small corrections |
| Magnesium sulfate heptahydrate (Epsom, reef grade) | 10.14 mg | Adds sulfate; don't use alone for big or repeated corrections |
| Mix of both, or a commercial Mg product | Varies | Keeps chloride:sulfate ratio closer to seawater |
Use the Coral Compass calculator to get grams for your volume. Raise by no more than about 100 ppm per day, dissolve in RO/DI first, and add slowly to high flow. Mg products are typically bulky; a 100 ppm raise in a 75-gal tank takes roughly 240 g of MgCl₂·6H₂O.
Maintenance
Regular water changes with a quality salt usually maintain Mg. 2-part users can add a Mg part (Recipe 1 has a magnesium component). Test Mg every 2–4 weeks, or weekly when correcting.
Sources
- Randy Holmes-Farley, "Do-It-Yourself Magnesium Supplements for the Reef Aquarium," Reefkeeping Magazine
- Randy Holmes-Farley, "A Simplified Guide to the Relationship Between Calcium, Alkalinity, Magnesium and pH," Reefkeeping Magazine
FAQ
Can high magnesium hurt? Modestly high (1400–1500 ppm) is generally tolerated. Raising it far above natural seawater offers no benefit.
Does magnesium fix bryopsis? Some reefers raise Mg with specific products to fight Bryopsis; the effect is tied to impurities in certain products, not magnesium itself. Treat with caution.
Should I fix magnesium before alkalinity? If Mg is very low (under 1100 ppm), correct it first; alk and Ca are easier to hold once Mg is normal.