Why does my magnesium keep dropping?

Coral and coralline algae incorporate Mg into their skeletons (about 4 mol% of calcite). Water changes replenish only what's in the salt mix — most salt brands target ~1,300 ppm. If you're consuming 30 ppm/week and changing 10 % weekly, you're net-negative. Dose, don't wait for water changes.

Is Epsom salt alone fine?

For a one-time correction, yes. For ongoing maintenance, no — you'll build sulfate over months. Use Recipe 3A or a balanced product (Foundation C, Tropic Marin Bio-Mg).

Can magnesium be too high?

Above 1,600 ppm starts causing dosing pump and probe issues, plus salt mixes that come in already-high (Red Sea Coral Pro) can drift further if you dose without testing. 1,400 is the practical ceiling.

Calculator

Magnesium Dosing Calculator

Magnesium gets tested less often than alk or Ca and corrected with too-aggressive single doses when it does get tested. Target 1,250–1,400 ppm. Below ~1,150 the calcium-carbonate solubility equilibrium starts working against you and dosing alk drives Ca precipitation instead of coral growth. Randy Recipe 3A blends MgCl₂·6H₂O with Epsom salts (MgSO₄·7H₂O) in a ratio that approximates natural seawater chloride/sulfate (~7:1).

Magnesium dosing

Tank volume (US gal)

Product / recipe

Current Mg (ppm)

Target Mg (ppm)

Required change

100.0ppm Mg

Liquid dose

402.70mL

5 cups MgCl₂·6H₂O + 3 cups Epsom salts (MgSO₄·7H₂O) dissolved to 1 US gal. The chloride/sulfate ratio approximates NSW (7.1:1) and avoids long-term sulfate buildup.

Source: reefkeeping.com

How this is calculated

Recipe 3A stock: 5 cups MgCl₂·6H₂O + 3 cups MgSO₄·7H₂O → 1 gal
                 ≈ 47,000 ppm Mg = 47 mg Mg / mL

mL = tank_gal × ΔMg_ppm × 3.785 / 47
   ≈ tank_gal × ΔMg_ppm × 0.0805

The chloride/sulfate ratio matters more than people think. Pure MgSO₄ (Epsom salt) is cheap and pulls Mg up fast, but long-term it strips natural sulfate ratios. Pure MgCl₂ goes the other way — chloride creep. Randy's 3A blend matches roughly the NSW ratio so you can dose long-term without skewing the salt mix.

FAQ

Why does my magnesium keep dropping?: Coral and coralline algae incorporate Mg into their skeletons (about 4 mol% of calcite). Water changes replenish only what's in the salt mix — most salt brands target ~1,300 ppm. If you're consuming 30 ppm/week and changing 10 % weekly, you're net-negative. Dose, don't wait for water changes.
Is Epsom salt alone fine?: For a one-time correction, yes. For ongoing maintenance, no — you'll build sulfate over months. Use Recipe 3A or a balanced product (Foundation C, Tropic Marin Bio-Mg).
Can magnesium be too high?: Above 1,600 ppm starts causing dosing pump and probe issues, plus salt mixes that come in already-high (Red Sea Coral Pro) can drift further if you dose without testing. 1,400 is the practical ceiling.

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Sources & references

01
Randy Holmes-Farley — Recipe 3A (Reefkeeping, July 2006)
https://reefkeeping.com/issues/2006-07/rhf/index.php
02
Bulk Reef Supply — BRS Pharma Magnesium dosing
https://www.bulkreefsupply.com/content/post/brs-pharma-magnesium-dosing-instructions
03
Red Sea — Foundation Complete Manual
https://redseafish.com/wp-content/uploads/2020/11/24653_Manual-Foundation-Complete-GB-_v21a-WEB.pdf
04
Triton — CoreChem MgCl₂
https://www.triton.de/en/products/corechem

Last reviewed 2026-05-25