To raise magnesium in a reef tank, dose magnesium chloride hexahydrate dissolved in RO/DI water. Use mostly chloride with a small share of magnesium sulfate, or a commercial magnesium supplement. It takes about 158 grams of magnesium chloride hexahydrate to lift 100 gallons by 50 ppm, so corrections are measured in handfuls, not pinches. Aim for roughly 1,250-1,400 ppm and spread large increases over several days.
What should magnesium be in a reef tank? #
Natural seawater carries about 1,280-1,285 ppm magnesium. Chemist Randy Holmes-Farley’s reef parameters table recommends 1,250-1,350 ppm. In his magnesium article he says 1,250-1,400 ppm is fine for practical purposes and that slightly outside that range is likely acceptable too. Our reef tank parameters chart shows it alongside the other targets.
Magnesium isn’t consumed in large amounts the way calcium and alkalinity are, so it changes slowly. That’s good and bad. It rarely crashes overnight, but a slow decline over months is easy to miss if you only test calcium and alk.
Why does magnesium matter if corals barely use it? #
Magnesium’s main job in a reef tank is chemical, not biological. Seawater is supersaturated with calcium carbonate, meaning calcium and carbonate would happily precipitate as limestone if nothing stopped them. Magnesium binds to the surface of calcium carbonate crystals as they start to form and blocks them from growing.
When magnesium runs low, that protection weakens. Calcium and alkalinity both start falling faster than your corals can account for. You’ll see crusty white deposits on heaters and pumps, and you’ll dose more and more with nothing to show for it. If you’re struggling to keep calcium and alkalinity up at the same time, test magnesium first.
How much magnesium chloride raises magnesium by 50 ppm? #
Magnesium chloride hexahydrate (MgCl2·6H2O) is about 12% magnesium by weight. Raising one US gallon by 1 ppm takes 3.785 mg of magnesium, which is about 31.7 mg of the hexahydrate. Use your net water volume: tank plus sump, minus rock and sand.
| Net water volume | MgCl2·6H2O for +50 ppm | For +100 ppm |
|---|---|---|
| 20 gal (76 L) | 32 g | 63 g |
| 40 gal (151 L) | 63 g | 127 g |
| 75 gal (284 L) | 119 g | 238 g |
| 100 gal (379 L) | 158 g | 317 g |
| 150 gal (568 L) | 238 g | 475 g |
Those are big numbers compared with alkalinity or calcium doses. That’s normal. Magnesium is present at around 1,300 ppm, so moving it a few percent takes a lot of product.
Why not use Epsom salt? #
Epsom salt is magnesium sulfate heptahydrate. It works, but it’s only about 10% magnesium, so you need more of it (about 38 mg per gallon per ppm). It also adds a lot of sulfate. Using it alone for large or repeated corrections pushes sulfate out of balance with seawater over time.
The approach Randy recommends is mostly magnesium chloride with a smaller amount of magnesium sulfate. That roughly matches the ratio of chloride and sulfate the tank loses. His DIY two-part article includes a magnesium part built that way. For a one-off correction, magnesium chloride alone is fine.
Commercial magnesium supplements #
Red Sea Reef Foundation C (liquid) raises 100 liters by 1 ppm per ml, according to the maker. That’s about 189 ml to raise 100 gallons by 50 ppm. Other brands differ, so dose from the label on your bottle.
How do you raise magnesium step by step? #
- Test twice. Magnesium kits require a lot of drops and careful color reading, and results can vary by 50 ppm or more between tests. Average two readings before acting.
- Decide the target. If you’re at 1,150 ppm, aim for 1,300 ppm, not 1,450.
- Calculate the dose from the table above or your product label.
- Dissolve it fully in RO/DI water. Magnesium chloride flakes dissolve easily. A mild warmth is normal.
- Split large corrections. Divide an increase of 150 ppm or more over three or four days.
- Add it slowly to the sump or a high-flow area.
- Retest after a day, then again before the next dose.
Store magnesium chloride in an airtight container. It pulls moisture from the air and turns into a wet clump if left open.
Why does magnesium drop in a reef tank? #
- Coral and coralline growth. Magnesium gets incorporated into skeletons in small amounts, especially by coralline algae.
- Two-part dosing without a magnesium part. Dosing calcium chloride and baking soda for months, with no magnesium, slowly shifts the ratio.
- Precipitation. Kalkwasser overdoses or very high pH can take magnesium out along with calcium carbonate.
- A low-magnesium salt mix. Some salts mix up below 1,300 ppm. Test a freshly mixed batch to know what your water changes are adding.
How do you keep magnesium stable over time? #
Once magnesium is back in range, the goal is to stop correcting it at all. How you do that depends on how you supply calcium and alkalinity.
| Setup | How magnesium gets replaced | What to watch |
|---|---|---|
| Regular water changes only | New salt carries magnesium | Your salt’s magnesium level after mixing |
| DIY two-part | A third, magnesium part dosed in proportion to the other two | Stick to the recipe’s ratio |
| Commercial two-part | Some brands include a magnesium part, some don’t | Check the label, and add Mg separately if needed |
| Kalkwasser | Supplies no magnesium | Plan on separate magnesium dosing |
| Calcium reactor | Some media contains magnesium | Test every few weeks until you know the trend |
Randy’s DIY system handles magnesium with a third part dosed in a fixed proportion to the calcium and alkalinity parts, so magnesium keeps pace with consumption without separate testing and correcting. If you use any other method, a small weekly magnesium dose based on your measured decline does the same job.
A sudden big drop in magnesium is rarely real. Magnesium moves slowly, so a reading 100 ppm lower than last week usually means a test error. Retest before dosing.
How often should you test magnesium? #
Every one to two weeks is typical for an established reef, more often while you’re correcting it. Because magnesium moves slowly, a chart of four or five readings tells you far more than one test.
Fish Tank Tracker plots magnesium against a 1,300-1,400 ppm band alongside calcium and alkalinity, so a slow slide or a correction that overshot shows up clearly. You can add your magnesium test kit to the equipment log and set a recurring task every 14 days, and the app sends a local notification when it’s due. Everything stays on your phone, with no account.
Frequently asked questions #
Can magnesium be too high in a reef tank? #
Randy’s guidance treats a range around 1,250-1,400 ppm as fine and slightly outside it as likely acceptable. Very high levels waste supplement and can affect some invertebrates. If you’ve overshot, water changes will bring it down gradually.
Does magnesium raise alkalinity? #
No. Magnesium supplements don’t add alkalinity. What they do is help calcium and alkalinity stay in solution, so fixing low magnesium often stops alkalinity from falling so quickly.
How long does it take magnesium to mix in? #
Once dissolved, it spreads through the system as fast as your flow moves water. In most tanks a few hours is enough, and testing the next day gives a reliable reading.
Why won’t my magnesium go up after dosing? #
The most common reasons are test error, dosing based on the tank’s full size rather than net water volume, or a miscalculated dose. Retest with fresh reagents, and double-check your math before adding more.
Is Epsom salt safe for a reef tank? #
In small amounts, yes. Epsom salt is magnesium sulfate, and some sulfate belongs in seawater. The problem comes from using it alone for big or ongoing corrections, which raises sulfate beyond natural levels.