How much nitrate and phosphate does a planted tank need?
There is no single magic number. What matters is that plants never run short, while levels stay well below what stresses fish.
Typical ranges
| Tank type | NO₃ in water | PO₄ in water |
|---|---|---|
| High light + CO₂ (EI) | 10–30 ppm | 1–3 ppm |
| Medium light, lean dosing | 5–20 ppm | 0.5–2 ppm |
| Low-tech, no CO₂ | 5–10 ppm | 0.2–1 ppm |
These are levels in the water, not amounts added. The amount you need to add each week is smaller than the level it settles at, because water changes remove a share. See how water changes affect dosing.
The N:P ratio
Many EI-style plans add roughly ten parts nitrate to one part phosphate by weight (for example 22.5 ppm NO₃ and 3 ppm PO₄ a week, about 7.5:1). Ratios between about 5:1 and 15:1 are commonly used. Very low phosphate with high nitrate is a frequent cause of green spot algae and stunted growth.
From ppm to grams
Potassium nitrate is 61.3 % nitrate by weight, so 1 g dissolved in 100 L adds 6.13 ppm NO₃. Monopotassium phosphate is 69.8 % phosphate, so 1 g in 100 L adds 6.98 ppm PO₄. The calculator does this for any volume and splits the weekly amount into doses: try 15 ppm NO₃ and 1.5 ppm PO₄ per week in 100 L.
Fish safety
Nitrate in the tens of ppm is generally tolerated by community fish, but very high levels stress livestock. The calculator warns when a single dose or the settled level looks extreme. If your tap water already contains nitrate or phosphate, subtract it from your targets.
Using liquid products?
Seachem Flourish Phosphorus states that its beginner dose raises phosphate by about 0.15 mg/L and that ideal levels "generally range from 0.15–1.0 mg/L" for its approach, which is much leaner than EI. Both approaches work in the right tank. Compare doses in the product library.
FAQ
Is 40 ppm nitrate too high for a planted tank?
Open the dosing calculatorMore guides
Sources
- Barr Report: The Estimative Index of Dosing
- Barr Report: EI target ranges
- Seachem Flourish Phosphorus: directions and phosphate range
- Potassium nitrate: molar mass
- IUPAC standard atomic weights
Written in our own words from the sources above. Numbers are educational starting points; check product labels and your water.