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

Name: Magnesium Dosing Calculator
Author: reefcalcs

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 — see the magnesium reference for the full range discussion. Below ~1,150 the calcium-carbonate solubility equilibrium starts working against you and dosing alkalinity 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).

reefcalcs editorialUpdated May 25, 2026Reviewed for accuracy May 15, 20264 sources cited

JavaScript is disabled in your browser, so the interactive calculator below won't run. The math, formula, and worked example are all written out further down this page under “How this is calculated.” Enable JS (or open this page in another browser) to use the live calculator.

Whether you compute by hand or in the calculator, the same caveat applies: verify against your test kit and the linked primary source before dosing. reefcalcs is reference information, not veterinary or engineering advice.

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 much MgCl₂ that actually is

Magnesium Chloride hexahydrate flakes — 5 lb

Burning ~240 g/correction, one 5 lb pack covers ~9.4 corrections · ~$25.

Based on a 100 ppm magnesium correction on a 75 gal (284 L) tank, at 0.836 g of MgCl₂·6H₂O per 100 L per ppm. Run your own numbers above — demand scales with tank volume and coral load.

Hexahydrate flakes are the form every DIY magnesium recipe is written around — buy this rather than an unlabelled powder, or your dose math will be off by a third.

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Gear for this

BRS Pharma Magnesium Mix ~$25 · View on Amazon →

Salifert Magnesium test kit ~$25 · View on Amazon →

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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.

Where to buy

Salifert magnesium is the standard hobby test kit. ESV B-Ionic includes a magnesium component if you want a complete pre-mixed three-part.

Salifert Magnesium test kit

Test monthly. Mg drift is the silent cause of Ca/alk drops.

View on Amazon →

Red Sea Reef Foundation Pro test kit

Ca, Mg, alk in one box. Designed for Red Sea's reef recipes.

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ESV B-Ionic Two-Part

Pre-mixed liquid two-part if you don't want to DIY.

View on Amazon →

Affiliate links. As an Amazon Associate, reefcalcs earns from qualifying purchases.

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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