Magnesium (Mg)
The smaller half of general hardness, the centre atom of chlorophyll, and the parameter most planted-tank deficiencies actually trace back to.
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.
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.
How to test
JBL Magnesium, Salifert Mg, or calculated from GH minus calcium. Calculated values are accurate enough for planted-tank dosing.
What high and low look like
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. Above 30 mg/L in freshwater: usually harmless, but pushes the GH high enough that soft-water fish may sulk.
How to fix
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.
