Magnesium (Mg)

Planted tanks want magnesium at 5–15 mg/L, roughly a quarter to a third of the calcium level. The tell-tale deficiency is yellowing between the veins of older leaves while new growth stays green — a pattern beginners routinely treat as iron deficiency. The fix is cheap: 1 g of Epsom salt per 100 L raises Mg by about 1 mg/L.

What it is

Magnesium is the second divalent cation that contributes to general hardness, typically sitting at a quarter to a third of the calcium concentration in unmodified tap water. In planted tanks it's the central atom of every chlorophyll molecule — without it, plants literally cannot photosynthesise.

On the GH scale it also punches above its weight: one degree corresponds to about 4.3 mg/L of Mg versus 7.1 for Ca, so milligram for milligram, magnesium moves GH roughly 1.6 times as hard. Which is exactly why a modest Mg imbalance hides so easily inside a normal-looking GH reading.

Why it matters

Plants and invertebrates compete for the Ca/Mg pool from the same water column. Tap water with a strong Ca:Mg imbalance — common in chalk aquifers where calcium dominates 8:1 or higher — starves plants of magnesium even when GH reads "fine". The classic symptom is yellowing between the veins of older leaves (interveinal chlorosis), which the unwary will treat as an iron deficiency and over-dose Fe instead.

The way to untangle the two is mobility. Magnesium is a mobile nutrient — a starving plant strips it out of old leaves and ships it to the growing tip, so the damage shows on older leaves first. Iron is immobile — a plant can't relocate it, so iron chlorosis bleaches the newest leaves. Same yellow, opposite ends of the stem. That single distinction settles most "my plants are turning yellow" threads before anyone reaches for a test kit.

How a deficiency plays out

Below 5 mg/L: interveinal chlorosis on older plant leaves, slow growth, weak response to other fertilisation. The classic "I'm dosing NPK and the plants still aren't growing" is most often a magnesium deficiency.

Watch a single leaf and the progression is characteristic: the tissue between the veins pales from green to yellow while the veins hold their colour, then the palest patches thin out and turn transparent, and finally the leaf melts. Fast growers show it first — Hygrophila, Limnophila, the big sword plants — because they're moving the most magnesium into the most new growth. Red plants wash out toward pale pink before the chlorosis pattern even becomes obvious. Livestock is far more patient: magnesium is an enzyme cofactor in moulting, but shrimp only run into trouble under extreme Ca:Mg skew — north of 10:1 — which is why caridina keepers watch the ratio rather than the absolute number. Fish are effectively indifferent at freshwater concentrations.

Too much magnesium

The 15 mg/L ceiling on the target band is a drift flag, not a danger line. Above 30 mg/L in freshwater: usually harmless, but pushes the GH high enough that soft-water fish may sulk. The genuinely unwanted effects arrive by accumulation: Epsom salt doesn't evaporate, doesn't precipitate and is consumed only slowly, so repeated "tonic" doses stack up until only water changes bring the number back down. And the competition works in both directions — very high Mg can crowd out calcium uptake in plants just as excess Ca crowds out Mg. If your GH creeps upward while calcium tests flat, magnesium is where the creep lives.

Raising and lowering it

Too low — Epsom salts (magnesium sulfate, MgSO₄·7H₂O) are cheap and effective. 1 g per 100 L adds ~1 mg/L Mg. Aim for a 4:1 Ca:Mg ratio for general planted tanks, 3:1 for high-tech ones. Most remineralisers (Equilibrium, Salty Shrimp GH+) already include Mg in the right proportion if you trust the ratio.

The arithmetic is friendly: to lift a 100 L tank from 3 to 8 mg/L, dissolve 5 g of Epsom salt in a jug of tank water and pour it in — no need to spread it over days, since the osmotic effect of 5 mg/L is trivial. The one mistake worth avoiding is fixing a ratio problem with a full remineraliser: GH salts raise calcium and magnesium together, so a tank that's Ca-rich and Mg-poor just gets harder while the ratio barely moves. Dose Mg alone, and let the dosing calculator turn grams into a target. Lowering follows the rule of all hardness: dilute with RO across normal water changes. Hardscape is rarely the culprit here — limestone is nearly pure calcium carbonate — with dolomite as the notable exception, which leaches Ca and Mg both.

Testing notes

JBL Magnesium, Salifert Mg, or calculated from GH minus calcium. Calculated values are accurate enough for planted-tank dosing.

The freshwater Mg drop kits are titrations with a resolution of a few mg/L per drop — workable at a 5–15 mg/L target, unlike the reef-scaled calcium kits. The calculated route is often just as good: take a careful GH reading, subtract the calcium, and the hardness converter turns the remainder into mg/L of Mg. Two habits keep the numbers honest. Test monthly rather than weekly — magnesium moves slowly, and one logged monthly value shows the trend better than a stack of noisy weekly ones. And retest your tap a couple of times a year: utilities blend sources seasonally, and an autumn switch can quietly halve the magnesium your water changes deliver.