How to Lower Magnesium in a Reef Tank
Magnesium is rarely brought down by adding something. It comes down by confirming the reading, finding what is putting it in, and letting routine water changes do the work.
The short answer
There is no additive that removes magnesium, and no safe way to strip it out quickly. Magnesium leaves a reef tank in only two ways: the tank consumes it, mainly through coralline algae and coral calcification, and you replace water that contains it. So a high reading is corrected from the input side — stop what is adding magnesium, check that your salinity is right, and keep your normal water-change routine. Expect weeks, not days.
- Check salinity first — A salinity error raises every ion at once, magnesium included.
- Confirm the test — Magnesium kits are the least precise of the common reef tests.
- Stop the input — A supplement, a dosing schedule, or a salt mix that already runs high.
- Then wait — Consumption and routine water changes, in that order, over weeks.
A fast drop in magnesium is riskier than the reading that prompted it, because the changes that force it down — big water changes, sudden salinity swings, a new salt mix — disturb everything else in the water at the same time.
Before you trust the number
Most high magnesium results are one of three things: a real input problem, a salinity error, or a test that is not reading what you think it is. Rule out the last two first — it costs you ten minutes and saves you a week of changes.
Test the same sample twice
Same sample, same time of day, in-date reagent. If the two readings disagree, the kit or the technique is the problem, not the tank.
Check the kit against its own reference standard
Most magnesium kits ship with a reference solution. If the kit cannot read its own standard correctly, it cannot read your tank correctly.
Check salinity before you check anything else
Magnesium scales with salinity. Confirm specific gravity with a calibrated instrument, and calibrate the refractometer with the solution it was made for rather than with tank water.
Check what you top off with
Evaporation removes water, not salt. Topping off with saltwater instead of fresh water concentrates every ion in the tank, magnesium included, and it is one of the most common causes of a reading that climbs on its own.
Write the number down
Record the reading, the date, the time, the test method and anything that changed that week. Two or three entries turn a single number into a trend you can act on.
Do not act on one reading
A single result, or a colour matched under warm room lighting, is not a diagnosis. Confirm it, then decide.
Match what you measured to the first move
Find the row that describes your tank and start there. Every row has one first move and one thing to avoid.
| What you measured | What it usually means | First move | What not to do |
|---|---|---|---|
| Magnesium high and salinity high | A salinity error raises every ion at once. Magnesium is following the salinity, not leading it. | Bring salinity back to your normal target with fresh-water top-off, then re-test both. | Do not dose or add anything to correct magnesium. |
| Magnesium high, salinity normal, a supplement or dosing pump running | The input is exceeding what the tank consumes. | Stop the magnesium input, note the date, and re-test on your normal schedule. | Do not remove tank water to speed the drop up. |
| Magnesium high, salinity normal, nothing being dosed | Either the reading or the salt mix — the tank cannot manufacture magnesium. | Verify the kit against its reference standard, then look at the mix you use for water changes. | Do not switch salt mixes in one large change. |
| Two kits give two different numbers | Different reagents, resolutions and reference standards are not comparable. | Pick one method and stay with it, then judge the trend rather than the number. | Do not average two kits into one reading. |
| Magnesium stable and slightly above your target | The tank is sitting where your salt mix puts it, and consumption is matching input. | Keep your routine water changes and stop chasing the number. | Do not treat a stable reading as an emergency. |
| Magnesium climbing with no change in dosing | Usually evaporation and top-off — salt does not evaporate with the water. | Check what you top off with, re-check salinity, and re-test magnesium afterwards. | Do not assume the tank is producing magnesium. |
Where the magnesium is coming from
Magnesium enters a reef tank in fewer ways than most keepers expect. Work down this list and you will usually find yours.
A magnesium supplement or a dosing schedule
The most common cause of a genuinely high reading. If a pump, a manual dose or a three-part system is adding magnesium, the tank only falls behind when consumption is lower than the schedule assumes.
InputThe salt mix you use for water changes
Every mix carries its own magnesium content, and mixes differ. If your mix runs high, routine water changes hold the number up rather than bringing it down.
InputTop-off water
Fresh water adds no magnesium. Saltwater used as top-off adds all of it, along with every other ion, and quietly raises salinity as it goes.
InputAll-in-one or trace supplements
Some products carry magnesium alongside other elements. Read the label of everything going in, not just the bottle marked magnesium.
InputSkimmers, carbon and phosphate media
These do their own jobs and do not selectively remove magnesium. Expecting them to lower it leads to running them harder than the tank needs.
Not a routeWater changes with the same mix
A water change only lowers magnesium if the new water carries less of it than the tank does. With a high-magnesium mix, routine changes hold the reading steady.
Not a routeTopping off with saltwater instead of fresh water raises salinity and magnesium together, slowly, over weeks. If your magnesium has crept up while nothing else changed, check this before you check anything else.
Read your number against the scale
These are the ranges reef references and hobby test kits commonly quote. They are waypoints for a decision, not a diagnosis — confirm any figure against your own kit's reference standard before you act on it.
Lower than the range most references quote for natural seawater. Magnesium is being consumed faster than it is replaced — this page is about the opposite problem.
The band most reef references and hobby kits treat as the working target. There is nothing to lower here.
Above the common target, but a band many mixed reefs run without incident. Confirm the reading and the salinity before changing anything.
Uncommon from consumption alone. Treat it as a measurement or salinity question first, and as a dosing emergency second.
| What the log shows | What it usually means | Where to go |
|---|---|---|
| Stable, week after week, with no dosing | Input and consumption have settled where your routine puts them. | Stop chasing the number. |
| Climbing steadily | Something is going in faster than the tank uses it, or salinity is creeping up. | Top-off and input audit. |
| Drops after a water change, then returns to the same level | The water you are adding carries about as much magnesium as the tank does. | Look at the salt mix. |
| Falling slowly while you dose nothing | The tank is consuming it normally, which is what magnesium demand looks like. | Keep testing, do not add anything. |
Which tank are you looking at?
Four common situations. Start with the one that matches yours.
The reading is high and so is the salinity
Fix one thing: bring salinity back with fresh-water top-off, and leave magnesium alone until the salinity has been stable for a few days. Magnesium usually follows on its own.
Go to the stepsSalinity is right and a supplement is running
This is the straightforward case. Stop the input, change nothing else, and let routine water changes and consumption do the work over the next few weeks.
Read the mistakesSalinity is right, nothing is dosed, and the number is still high
The two suspects left are the test kit and the salt mix. Verify the kit against its own standard first — it is free, and it is the faster answer.
Read the edge casesI do not know why it is high
Start with the log, not the tank. Test the same way for two or three weeks and the direction of the trend will tell you which of the six inputs to look at.
See the reading chartThe order that works
Work these in sequence. Each step exists to stop the next one from making things worse.
Step 1 — Confirm the reading
Test the same sample twice, at the same time of day, with in-date reagent, and check the kit against its own reference standard. If the kit cannot read its own standard, nothing downstream is worth acting on.
act on a single reading, or on a colour you matched under warm room lighting.
Step 2 — Check salinity and your top-off water
Confirm specific gravity with a calibrated instrument, and confirm that what you top off with is fresh water. A salinity error moves magnesium, calcium and alkalinity together, so correcting it changes three readings at once.
correct salinity and magnesium in the same week, and do not top off with saltwater to "keep the level up".
Step 3 — Stop the magnesium input
Turn off the supplement, the dosing pump channel or the manual dose, and write down the date you stopped. Read the labels of everything else you add, in case magnesium is arriving inside another product.
stop the input and add something else to counteract it in the same week.
Step 4 — Start a log with the numbers you already have
Write down the reading, the date, the time, the method and anything that changed that week. Two or three weeks of entries turns a single number into the trend that tells you whether stopping the input worked.
reconstruct the log from memory later, or mix results from two different kits in one column.
Step 5 — Keep your routine water changes, and only then look at the mix
Routine changes with a mix that carries less magnesium than your tank will bring the reading down slowly. If your mix already runs high, plan the change over several normal water changes rather than one large one, and mix the new salt to the same salinity and temperature you normally use.
run a large corrective water change, switch mixes in a single change, or exceed any manufacturer's own instructions.
Step 6 — Re-test on a fixed schedule and hold the trend
Pick an interval and keep it, testing the same way each time. Stop when the trend flattens: a stable number in the working range is the outcome, and it does not have to be a specific figure.
declare victory on one good reading, and do not keep pushing once the trend has flattened.
Paling tissue, sloughing skin, a sudden loss of colour, or a jump in alkalinity after a change means back the change off. Stop the last thing you did, re-test salinity and alkalinity, and let the tank settle before resuming more slowly.
Mistakes that make it worse
All six of these are common, and all six turn a manageable reading into a stressed tank.
Assuming the number is real
Magnesium is the least precise of the common reef tests, and it is the one most likely to be affected by a salinity error. Verify before you correct.
Topping off with saltwater
It raises salinity and every ion with it, and the change is slow enough that it is usually noticed only after weeks of rising readings.
Running a large water change to force it down
A big change moves salinity, alkalinity, calcium and temperature at the same time. The corals feel the swing long before the magnesium number moves.
Switching salt mixes in one change
A sudden change of mix changes the balance of everything in the water. Move between mixes over several routine water changes, not one.
Adding something to counteract it
There is no additive that removes magnesium. Adding a second product to fix the first one adds a variable you cannot measure.
Chasing a number instead of a trend
The reading moves for reasons that have nothing to do with your last change. A trend measured the same way each week is the only reliable signal.
Edge cases worth knowing
Six situations where the standard advice needs adjusting.
My kit says high and the shop's kit says normal.
Salinity was high and I have already corrected it.
My salt mix already runs high in magnesium.
I overdosed magnesium and I do not know how much went in.
My tank is soft corals and LPS only.
My tank is small, under about 30 gallons.
Questions we get about magnesium
Short answers to the questions that come up most often.
What is a normal magnesium level in a reef tank?
Will a water change lower magnesium?
Does anything remove magnesium from the water?
How fast can I bring magnesium down?
Is high magnesium dangerous to corals?
Why does magnesium read high again after every water change?
Where to go next
If you want to keep going, these are the parts of the site that pick up where this page stops.
Keep the log going
A reading without a record is just a number. Log magnesium, alkalinity and salinity the same way each week, and the trend will tell you what to change next.