Magnesium in a Reef Tank: Why It Matters and How to Keep It Stable

Magnesium is the quiet member of the big three. Alkalinity swings in days. Calcium drifts over weeks. Magnesium can sit still for a month and then, once it is low, make every other number hard to hold. Many keepers chase a stubborn alkalinity for weeks before they test magnesium at all.

What magnesium does in a reef tank

Seawater is already close to saturated with calcium carbonate. Left alone, calcium and carbonate ions want to bind and fall out as a white solid. Magnesium ions get in the way. They sit on the surface of tiny crystals as they form and slow their growth. This keeps calcium and alkalinity dissolved, available to your corals, and out of your pumps, heater and glass as crusty deposits.

Magnesium also takes part in biology. Corals and coralline algae take up a small amount into their skeletons, and it is a normal part of natural seawater. For the full picture of how the three work together, read the alkalinity, calcium and magnesium balance guide.

Target range and what low or high looks like

Natural seawater at 35 ppt holds roughly 1280–1300 ppm, so the target simply mirrors the ocean. Magnesium scales with salinity. A tank kept at a lower salinity will read lower magnesium even if nothing is wrong, so check salinity first.

How often to test, and how reliable the test is

Magnesium is consumed slowly. Most tanks lose it mainly through coral growth, coralline algae and the odd bit of salt creep. A test every 2–4 weeks is enough for a stable tank. Test more often after you correct it, or if alkalinity and calcium stop behaving.

Be careful with the result. Magnesium test kits tend to be less precise than alkalinity kits, and a difference of 50–100 ppm between two tests can be kit noise rather than a real change. Use the same kit each time, read it the same way, and treat one odd number as a reason to retest, not to dose.

Why magnesium drifts

• Coral and coralline growth. A heavily stocked SPS tank uses more than a soft coral tank.
• Salt mix differences. Brands vary. Some mix to around 1250 ppm, others higher. Test a fresh batch of mixed saltwater before you trust it.
• Low salinity. Topping off with too much fresh water pulls every salt component down together.
• Precipitation. Alkalinity or calcium dosed too fast can pull magnesium out along with them.

How to raise magnesium safely

Magnesium is gentle on corals compared with alkalinity, but slow is still better. A common rule is to raise it by no more than about 50–100 ppm a day, and to retest before the next dose. Always follow the instructions on the product you use, and never dose into a tank you have not tested first.

1. Test magnesium, and confirm salinity is 35 ppt.
2. Work out the gap to your target. Use the product label to find the amount for your actual water volume, not the tank's advertised size.
3. Split the total into small daily steps. Add each step slowly, into good flow, ideally in a sump or high-flow area.
4. Retest after the last step, a few hours after dosing, and again the next day.
5. Stop when you reach the range. Do not aim for the top of it.

If it is too high

There is no quick fix, and none is needed for a modest overshoot. Stop adding magnesium, then let ordinary water changes bring it down over time. Check what your salt mix delivers, because a salt that mixes high will keep pushing it up.

Track it with everything else

One magnesium reading tells you little. A slow slide from 1330 to 1230 over three months tells you your corals are using more than you replace. Log magnesium on the same date as alkalinity and calcium, and you can see all three trends side by side. In ReefDeck you can record each full test, set your own range for magnesium, and watch the line before it becomes a problem. It shows your numbers; the decisions stay with you.