Potassium (K)
The third macronutrient. The one no GH or KH test measures, the one a freshwater tank can run completely empty on, and the one most cheap fertilisers underdose.
What it is
Potassium is the K in NPK. Plants use it for stomatal regulation, enzyme activation, and turgor — moving water and nutrients around the cells. Unlike Ca and Mg, it contributes nothing to general hardness and slips past the test kits most aquarists use.
Why it matters
Potassium is the macronutrient most aquarium tap water sources are short on, and the one cheap "complete" fertilisers tend to underdose because the per-litre bottle math works out unfavourably. A K-deficient planted tank looks healthy until growth slows, then leaf edges develop pinholes that resemble PO₄ deficiency — except PO₄ dosing doesn't fix it.
For the 80% of planted tanks running tap or remineralised RO without supplemental K, the limiting nutrient is almost always potassium.
How to test
JBL Kalium, Sera K — the kits are rarer and slower to read than NO₃/PO₄ kits. Most planted-tank keepers dose K against a target (10–20 mg/L) without testing every week.
What high and low look like
Below 5 mg/L sustained: pinhole damage on older leaves, slow growth, plants that won't respond to N or P dosing. Above 100 mg/L: usually harmless to fish but accumulates fast if you over-dose and skip water changes.
How to fix
Dose potassium sulfate (K₂SO₄) for clean K addition, or potassium nitrate (KNO₃) if you also want NO₃. 1 g K₂SO₄ per 100 L adds ~4.5 mg/L K. Most complete macro fertilisers (Aqua Rebell Makro Basic NPK, Tropica Specialised) include K in the right proportion — the dosing calculator does the bottle math.
