Phosphate (PO₄)
A macronutrient for plants, a fuel for algae, and a parameter people keep misreading as bad-when-non-zero.
What it is
Phosphate is one of the three macronutrients planted-tank fertilisation rotates around — nitrogen, phosphorus, potassium. Fish food, food waste, and dosed PO₄ are the main sources; plant uptake and binding to substrate are the main sinks.
Why it matters
Aquarium folklore from the 1990s held that any detectable phosphate was a problem because it "fed algae". That was a half-truth: algae grows when one of light, CO₂, or nutrients is the limiting factor — and in a CO₂-injected planted tank, starving the plants of PO₄ usually starves them faster than it starves the algae. Modern Estimative-Index and Tropica-style fertilisation runs PO₄ at 0.1–2.0 mg/L deliberately.
How to test
Liquid kits (JBL PO₄ Sensitiv, Salifert PO₄, Hanna ULR Checker for sub-ppm). Strip tests are too coarse below 1 mg/L. Test before a planned water change so the value reflects the cycle low; if you dose daily, test mid-cycle to see what your fertilisation routine actually maintains.
What high and low look like
Below 0.05 mg/L sustained: plant leaves develop pinholes, particularly on the older leaves of fast growers like Limnophila and Cabomba. New growth stays small. Above 3–5 mg/L: not directly harmful but pushes the nutrient profile toward easy algae growth if light and CO₂ aren't keeping pace.
How to fix
Too low — dose KH₂PO₄ (mono-potassium phosphate) directly, or a complete macro fertiliser containing phosphate. Aim for 0.5–1.0 mg/L weekly low after water change. The dosing calculator does the millilitre math. Too high — increase water-change percentage or reduce feeding; PO₄ binders are a last resort because they tend to overshoot.
