Why it matters
Low magnesium causes calcium and alkalinity to crash together. Hard to dose Mg back up but the keystone of stable reef chemistry.
Ideal ranges
| Water type | Ideal | Acceptable |
|---|---|---|
| Reef | 1280 to 1350 ppm | — |
| Fish-only saltwater | 1200+ ppm | — |
How to test
- 1
Salifert or Red Sea Magnesium Pro
Salifert is the most-used. Test monthly when dosing 2-part.
How to correct low magnesium
Dose magnesium chloride + sulfate (recipe)
Bulk Reef Supply sells the salts. 2:1 chloride to sulfate by weight.
Brightwell Magnesion or Tropic Marin Bio Magnesium
Pre-mixed liquid for convenience.
Common causes
- •Heavy coral growth depleting Mg
- •Salt mix that runs low magnesium
- •Long water change intervals
FAQs
Can I let magnesium drift below 1200 ppm?
Avoid it. Below 1200, calcium and alkalinity become unstable and harder to maintain. 1280 is a safe baseline.