How much magnesium chloride (or Epsom salt) raises magnesium 10 ppm?
8.4 g of magnesium chloride hexahydrate per 100 liters, or 10.1 g of Epsom salt — 10 ppm in 100 L is 1,000 mg of magnesium, magnesium is 24.305 g per mole, and the two salts weigh 203.3 and 246.47 g per mole with their water of hydration. The balanced blend for the same 10 ppm is 7.6 g of the chloride plus 0.95 g of Epsom. Magnesium is the biggest number on the test sheet, so the grams get large quickly: bringing a 100 gallon system up 150 ppm is well over half a kilo of salt.
| System water | MgCl₂·6H₂O only | Epsom only | Balanced blend |
|---|---|---|---|
| 10 gal · 38 L | 3.2 g | 3.8 g | 2.9 g + 0.4 g Epsom |
| 20 gal · 76 L | 6.3 g | 7.7 g | 5.7 g + 0.7 g Epsom |
| 30 gal · 114 L | 9.5 g | 11.5 g | 8.6 g + 1.1 g Epsom |
| 40 gal · 151 L | 12.7 g | 15.4 g | 11.5 g + 1.4 g Epsom |
| 55 gal · 208 L | 17.4 g | 21.1 g | 15.8 g + 2 g Epsom |
| 75 gal · 284 L | 23.7 g | 28.8 g | 21.5 g + 2.7 g Epsom |
| 100 gal · 379 L | 31.7 g | 38.4 g | 28.7 g + 3.6 g Epsom |
| 125 gal · 473 L | 39.6 g | 48 g | 35.9 g + 4.5 g Epsom |
| 150 gal · 568 L | 47.5 g | 57.6 g | 43 g + 5.4 g Epsom |
| 200 gal · 757 L | 63.3 g | 76.8 g | 57.4 g + 7.2 g Epsom |
| 300 gal · 1,136 L | 95 g | 115.2 g | 86.1 g + 10.8 g Epsom |
Per 10 ppm of magnesium; multiply for your target. Blend ≈ 8:1 by weight, magnesium chloride hexahydrate to Epsom salt, holding seawater's chloride:sulfate ratio. Dissolve in RO water; up to 100 ppm a day.
Magnesium chloride vs Epsom salt vs the blend: what each one adds
Every magnesium salt drags a partner ion in with it — chloride or sulfate — and seawater has a fixed ratio of the two: about 19,353 ppm chloride to 2,712 ppm sulfate, 7.1:1. Dose only one salt for long enough and you bend that ratio. How much? Here is the 100 gallon system raised 150 ppm each way, computed by this page's engine:
| Raise 150 ppm Mg with… | Grams | Adds chloride | Adds sulfate |
|---|---|---|---|
| Magnesium chloride only | 475 g | 438 ppm (+2.3%) | — |
| Epsom salt only | 576 g | — | 593 ppm (+21.9%) |
| Balanced blend | 430 g + 54 g | 397 ppm (+2%) | 56 ppm (+2%) |
Percentages are relative to reference seawater's own chloride and sulfate. Blend ratio derived from those figures (9.7 mol MgCl₂ per mol MgSO₄); Holmes-Farley's two published recipes sit either side of it.
Read the columns and the advice writes itself. Chloride is such a huge reservoir that even a big all-chloride correction moves it a percent or two — nothing. Sulfate is a small reservoir, and an all-Epsom correction pushes it up by a seventh in one go; do that a few times and you have a measurably odd ocean. So: the blend for routine use, magnesium chloride alone if that's what's on the shelf, and Epsom alone only for a one-off nudge. None of this is dangerous — water changes reset both ions — but it's why "just use Epsom salt" is the cheap answer and not the good one.
Randy Holmes-Farley's DIY magnesium recipes use roughly 10:1 and roughly 1.5:1 magnesium chloride to Epsom by weight, and both are fine. The seawater-matching ratio the calculator uses (8:1) sits between them. It doesn't matter much because both counter-ions are enormous compared with the magnesium you're adding: the point of blending is to avoid pushing sulfate up a lot, and any blend that is mostly chloride achieves that. Precision goes into the magnesium number; the ratio just needs to be sensible.
How fast can I raise magnesium?
Faster than anything else on the reef test sheet. Corals and fish are indifferent to magnesium in the range hobbyists work in, so the usual pace of about 100 ppm a day is about mixing, dissolving and precipitation, not toxicity. The 100 gallon system at 1150 ppm aiming for 1300 needs 150 ppm — 484 g of blend — which the calculator makes 2 days of 75 ppm. Dissolve each day's dose fully (magnesium chloride goes into solution easily; Epsom needs a stir), and add it to high flow. Expect a small cloud of calcium sulfate if you mix the two salts in one jug — it's the calcium impurity in the chloride meeting the sulfate; harmless, and the reason the blend hint says to dissolve them separately.
Why magnesium matters for calcium and alkalinity
Magnesium doesn't build coral skeleton; it stops the skeleton from building itself in the wrong place. Magnesium ions crowd the surface of every tiny calcium-carbonate crystal that starts to form in the water column or on a heater, and slow its growth enough that calcium and carbonate stay dissolved at levels that would otherwise precipitate. Let magnesium sag below about 1250 ppm and the effect weakens: alkalinity and calcium doses stop "sticking," the tests drift down faster than the corals can explain, and pumps grow crust. That's the whole reason the order of operations is magnesium first, then alkalinity, then calcium — and why the calcium calculator asks for your magnesium before it lets you chase calcium.
Magnesium is consumed slowly — a few ppm a week in most tanks — so once it's in the 1250–1350 window and your salt mix lands there too, monthly testing is plenty. Two exceptions: a new tank on a low-magnesium salt, and any time calcium or alkalinity misbehave for no reason. Natural seawater is about 1280 ppm; the hobby runs slightly high because the precipitation-suppressing effect is the point. Above roughly 1,500 ppm, stop and let it fall — there's no benefit up there.