Magnesium in Reef Tanks

This guide covers reef tank magnesium with practical steps for home aquariums. Magnesium keeps calcium and alkalinity in solution — low Mg causes white precipitation on heaters and stalled coral growth.

For related reading, see aquarium water chemistry basics, aquarium GH and general hardness and Reef Alkalinity (dKH) Guide.

Why Magnesium Matters

Magnesium prevents calcium carbonate from precipitating out of seawater. Without adequate Mg, alkalinity and calcium tests lie — additives vanish onto equipment.

Natural seawater runs near 1280 ppm magnesium; reef tanks should stay in the 1250–1350 ppm hobby range.

Signs of Low Magnesium

White flaky crust on heaters and pumps, difficulty maintaining calcium despite dosing, and dull coralline growth often trace to neglected magnesium.

Calcium and alkalinity may test fine while Mg sits at 900 ppm — always test the trio together monthly.

Raising Magnesium

Use dedicated magnesium chloride/sulfate blends per manufacturer instructions. Raise slowly — 50–100 ppm per day max on stocked tanks.

Large water changes with properly mixed salt reset all parameters if way out of line.

Relationship to Other Parameters

After correcting Mg, calcium and alkalinity dosing often 'suddenly work' again. Adjust two-part doses only after Mg stabilizes 48 hours.

Do not chase magnesium with untested DIY salts — purity matters on small volumes.

Maintenance Schedule

Test monthly on mature fish-only reefs; biweekly on stony coral tanks. Magnesium depletes slower than alkalinity but still drifts with water changes and export.

Log results alongside alkalinity — patterns reveal when coral load increased consumption.