If the magnesium level in your reef tank is too high, it can potentially lead to problems with coral health, inhibit calcium and alkalinity uptake, and stress invertebrates. Here's what you need to know and how to fix it:
🧪 Ideal Magnesium Levels
Target range: 1250–1350 ppm
Too high: >1500 ppm (some soft corals can tolerate up to ~1600 ppm, but it’s still not ideal long-term)
⚠️ Symptoms of High Magnesium
Poor coral growth or polyp extension
Snails and other inverts acting lethargic or dying
Reduced calcium/alkalinity uptake
Cloudy water (rare, in very high cases)
🔧 How to Reduce Magnesium Safely
Stop dosing magnesium
Cease any supplements containing Mg (like part of 2-part dosing or commercial reef additives).
Water changes
Use a salt mix with a lower Mg content (check labels!).
Large or frequent water changes (10–25% weekly) help gradually bring Mg down.
Dilution over time
If levels aren't dangerously high (e.g., <1600 ppm), natural consumption and water changes will lower Mg gradually.
Use a lower-Mg salt mix
Some salt mixes (e.g., Red Sea Blue Bucket, Instant Ocean) tend to have more moderate Mg levels than reef-targeted mixes.
⏱️ Don't rush it!
Rapid drops in Mg can stress corals and inverts just as much as high levels. Aim to lower Mg slowly—no more than 50 ppm per day.
📏 Double-check your test
Use a reliable test kit (Salifert, Red Sea, or Hanna are popular).
Retest to confirm levels before making big changes.