Potassium (K)

Planted tanks want potassium somewhere between 5 and 30 mg/L, and unlike nitrate or phosphate it rarely arrives by itself — most tap water carries only 2–5 mg/L, and fish food adds little. The signature deficiency is pinholes in older leaves. The fix costs pennies: 1 g of K₂SO₄ per 100 L adds ~4.5 mg/L of potassium.

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.

It does enter through the back door of other fertilisers, though: KNO₃ and KH₂PO₄ both carry potassium as the counter-ion, which is why tanks on a full macro regime are usually covered without anyone ever thinking about K — and tanks that get their nitrate from other sources quietly run dry.

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.

The pinhole pattern

Below 5 mg/L sustained: pinhole damage on older leaves, slow growth, plants that won't respond to N or P dosing.

The holes have a signature. They start as pinpoint yellow spots on older leaves, the centres die and drop out, and each hole keeps a thin yellow rim — that rim is what separates a potassium deficiency from snail grazing or mechanical damage, which punch clean-edged holes in otherwise green tissue. Fast growers announce it first: Hygrophila, water sprite and Amazon swords perforate within weeks, while slow plants like Anubias hold out for months. Left alone, the holes merge, the leaf margins yellow, and the plant sheds its older leaves entirely. Livestock, meanwhile, doesn't care — potassium has no meaningful toxicity to fish or shrimp at any concentration you could reach by dosing, which is why this is a plants-only parameter.

Where it comes from — and where it doesn't

Tap water: usually 2–5 mg/L, occasionally more in agricultural regions, never enough for a serious plant mass. Shrimp remineralisers are built around calcium and magnesium and carry potassium at most as a trace — an RO shrimp tank with plants is the textbook zero-potassium setup. Seachem Equilibrium is the exception: roughly a fifth potassium by weight, which quietly covers the tanks that use it. Root tabs feed roots, not the water column, so stem plants drawing from the water still starve above a fertilised substrate. If you run a low-tech tank without CO₂ and without column dosing, potassium is the first macro worth adding — the planted-without-CO₂ guide walks through the wider nutrient picture.

When it climbs

Above 100 mg/L: usually harmless to fish but accumulates fast if you over-dose and skip water changes.

Between 30 and 100 mg/L nothing visibly goes wrong — the band exists to flag accumulation, because potassium has no exit from a tank except the water change: it doesn't outgas like CO₂, doesn't precipitate, and plants take only what they need. A K line that climbs test after test means dosing exceeds uptake; cut the dose rather than chasing it with extra changes. Some keepers report sensitive stem plants (Rotala species are the usual suspects) stunting in very high K, blamed on calcium-uptake antagonism — the evidence is anecdotal and contested, but staying inside the 5–30 band sidesteps the argument entirely. And because tap is low in potassium, ordinary water changes are the correction: two 30% changes remove roughly half of any excess.

Dosing it

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.

For a concrete target: lifting a 100 L tank by 10 mg/L takes 2.2 g of K₂SO₄ — about half a teaspoon, dissolved in a jug first. KNO₃ delivers two macros at once (1 g per 100 L ≈ 3.9 mg/L K plus 6.1 mg/L NO₃), which is efficient when both are low and a problem when nitrate is already at its ceiling — check where your nitrate-to-phosphate balance sits before choosing the salt. There's no rush and no shock risk; potassium can be corrected in a single dose.

Testing — and why most people don't

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.

The common hobby kits are turbidity titrations rather than colour charts: you add reagent drop by drop until a mark under the vial disappears behind the forming cloud, and the drop count gives the concentration. That makes them slow, and it makes technique matter — same lighting, same viewing angle, no rushing the drops, because the endpoint creeps up rather than snapping. Resolution lands at a couple of mg/L at best. The pragmatic routine: dose calculated amounts weekly, test monthly to confirm the level isn't ratcheting upward, and log the monthly value — accumulation that's invisible between two readings is unmistakable as a six-month line.