Magnesium stability is one of the quiet heroes of a thriving reef aquarium. While calcium and alkalinity get most of the attention, magnesium quietly keeps both in balance and helps prevent nuisance issues like precipitation and wild parameter swings. Understanding how magnesium works and how to keep it stable will make your reef more resilient and easier to maintain.

Why Magnesium Matters in a Reef Tank

In natural seawater, magnesium typically sits around 1280–1350 ppm. In our aquariums, it plays three key roles:

• Buffers calcium and alkalinity: Adequate magnesium helps prevent calcium carbonate from precipitating out of solution, which keeps your calcium and alkalinity usable for corals and coralline algae.
• Supports coral growth: Stony corals, clams, and coralline algae all rely on stable magnesium to build strong skeletons.
• Stabilizes overall chemistry: When magnesium is low, you may find it nearly impossible to maintain consistent calcium and alkalinity levels, even with careful dosing.

If you’re fighting constant swings in calcium or alkalinity, checking magnesium is one of the smartest troubleshooting steps you can take. For a deeper dive into overall water chemistry, see our guide on reef alkalinity balance.

Target Levels and Testing Routine

For most mixed reef aquariums, a safe and effective magnesium range is:

• Target: 1300–1400 ppm
• Minimum: ~1250 ppm
• Do not exceed: ~1500–1550 ppm for long-term use

Magnesium doesn’t fluctuate as quickly as alkalinity, so you don’t need to test it daily. A good schedule for most tanks is:

• New tanks or heavy dosing: Test weekly until stable.
• Established tanks: Test every 2–4 weeks.
• Whenever parameters drift: Test magnesium if calcium or alkalinity become difficult to control.

Tip: Always test magnesium before making big changes to calcium or alkalinity. Correcting magnesium often makes the rest much easier to dial in.

Practical Strategies for Magnesium Stability

Keeping magnesium stable is less about chasing numbers and more about consistent habits. Focus on these areas:

1. Start with a Solid Salt Mix

Your salt mix sets the baseline for every water change. Different brands have different magnesium levels, so test a fresh batch of saltwater at your standard salinity (around 1.025–1.026 SG). If your salt consistently mixes at 1350 ppm, it will naturally support stable levels in the display tank.

For help choosing and mixing salt, check out our article on mixing saltwater for reef tanks.

2. Dose Slowly and Consistently

If testing shows your magnesium is low, raise it gradually. Large, fast corrections can stress corals. Aim to increase magnesium by no more than 50–100 ppm per day.

• Use a dedicated magnesium supplement (often a blend of magnesium chloride and magnesium sulfate).
• Re-test 24 hours after dosing to confirm the new level.
• Once you reach your target, track weekly consumption to set up a maintenance dosing schedule.

Many reefers use dosing pumps to automate small daily additions, which helps keep magnesium, calcium, and alkalinity all moving together in a stable pattern.

3. Watch for Hidden Magnesium Consumers

Some setups use more magnesium than others. You may see faster depletion if you have:

• Rapidly growing SPS and LPS corals
• Heavy coralline algae growth on rock and equipment
• Very high calcium and alkalinity levels (which increase overall demand)

If growth is strong and your tank is packed with calcifying organisms, expect to test and dose magnesium more frequently. Align your magnesium dosing plan with your calcium and alkalinity schedule so all three stay in sync. For a step-by-step approach, see our guide to reef parameter testing schedules.

Bringing It All Together

Stable magnesium is the quiet foundation of a healthy reef aquarium. By aiming for 1300–1400 ppm, testing on a regular schedule, and making slow, measured adjustments, you’ll find that calcium and alkalinity become easier to maintain and your corals respond with stronger growth and better coloration. Treat magnesium as a core part of your reef chemistry strategy, not an afterthought, and your tank will reward you with long-term stability and fewer surprises.

Sources

• Holmes-Farley, R. (Reefkeeping Magazine). “Magnesium in Reef Aquaria.”
• Spotte, S. “Seawater Aquariums: The Captive Environment.”
• Borneman, E. H. “Aquarium Corals: Selection, Husbandry, and Natural History.”