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 #3 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.

Magnesium Dosing Calculator — Randy Recipe #3, BRS, Tropic Marin, Red Sea

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

473.18mL

7¼ cups MgCl₂·6H₂O flake + ¾ cup Epsom salt (MgSO₄·7H₂O) dissolved in 1 US gal of water. Stock ≈ 40,000 ppm Mg; 1 mL per US gal raises Mg ≈ 10.6 ppm. The chloride/sulfate ratio (~7:1 by mass) matches NSW and avoids long-term sulfate buildup.

Source: reefkeeping.com

How much MgCl₂ that actually isAffiliate link

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.

View on Amazon →

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

Gear for thisAffiliate links

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

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

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

How this is calculated

Recipe #3 stock: 7¼ cups MgCl₂·6H₂O flake + ¾ cup MgSO₄·7H₂O
                 dissolved in 1 US gal water (≈ 4.8 L of stock)

  MgCl₂·6H₂O ≈ 1,458 g × 11.96 % Mg ≈ 174 g Mg
  MgSO₄·7H₂O ≈   186 g ×  9.86 % Mg ≈  18 g Mg
  ≈ 193 g Mg / 4.8 L ≈ 40,000 ppm Mg = 40 mg Mg / mL

mL = tank_gal × ΔMg_ppm × 3.785 / 40
   ≈ tank_gal × ΔMg_ppm × 0.095

Randy gives the recipe by volume; the gram figures use his stated bulk densities (0.85 g/cm³ for magnesium chloride flake, 1.05 g/cm³ for Epsom salt), and the ~40,000 ppm stock strength is our stoichiometry, not a number from the article. If you mix by weight, ~1,460 g of MgCl₂·6H₂O plus ~185 g of Epsom salt in a gallon of water gives the same stock.

The chloride/sulfate ratio matters more than people think. Pure MgSO₄ (Epsom salt) is cheap and pulls Mg up fast, but long-term it builds sulfate past natural ratios. Pure MgCl₂ goes the other way — chloride creep. Recipe #3 works out to ~7:1 chloride to sulfate by mass, 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 #3 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

Affiliate links

Salifert magnesium is the standard hobby test kit. For the dose itself: BRS Pharma Magnesium as a ready blend, MgCl₂·6H₂O flakes plus Epsom salt for Randy Recipe #3, or Red Sea Foundation C if you already run that program.

Salifert Magnesium test kitTest monthly. Mg drift is the silent cause of Ca/alk drops.View on Amazon →BRS Pharma Magnesium MixMagnesium chloride + sulfate blend for Randy Recipe 3 magnesium.View on Amazon →Magnesium Chloride hexahydrate flakes — 5 lbMgCl₂·6H₂O flakes. The bulk magnesium source for DIY dosing.View on Amazon →Red Sea Reef Foundation C (Magnesium) — 1 kg powderMagnesium component. Dose only when Mg tests low.View on Amazon →

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

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Sources & references

1.
Randy Holmes-Farley — Do-It-Yourself Magnesium Supplements, Recipe #3 (Reefkeeping, July 2006)
https://reefkeeping.com/issues/2006-07/rhf/index.php
2.
Bulk Reef Supply — BRS Pharma Magnesium dosing
https://www.bulkreefsupply.com/content/post/brs-pharma-magnesium-dosing-instructions
3.
Red Sea — Foundation Complete Manual
https://redseafish.com/wp-content/uploads/2020/11/24653_Manual-Foundation-Complete-GB-_v21a-WEB.pdf
4.
Triton — CoreChem MgCl₂
https://www.triton.de/en/products/corechem

Last reviewed 2026-09-30