Nitrogen, phosphorus and potassium in planted tanks

The three primary macronutrients

Plants need 17 essential elements. Six are macronutrients needed in large amounts: nitrogen, phosphorus and potassium, called primary because their deficiencies are the most common, and calcium, magnesium and sulfur, called secondary ^[1]. Tropica calls CO2 the main nutrient for aquarium plants and the main inhibitor of growth ^[3]. After carbon, N, P and K are the elements aquarium methods dose most.

Nutrient	What it does	Form plants take up
Nitrogen	proteins, nucleic acids (DNA, RNA) and chlorophyll	nitrate NO3⁻, ammonium NH4⁺
Phosphorus	ATP (energy), sugars, nucleic acids	phosphate H2PO4⁻ and HPO4²⁻
Potassium	enzyme activation, photosynthesis, protein formation, sugar transport	K⁺

Sources: functions ^[2]; uptake forms ^[1].

All three are mobile inside the plant: when supply runs short, the plant moves them from older leaves to new growth, so deficiency shows first in older, lower leaves ^[2]. The deficiency symptom key uses that rule.

A note on terminology: Tropica’s fertilizer guide lists nitrogen, phosphorus, iron and manganese as the macronutrients for aquarium plants ^[3]. Agronomy places iron and manganese among the micronutrients and calls N, P, K, Ca, Mg and S the macronutrients ^[1]. The grower’s list reflects what aquarists dose most, not the botanical classification.

How aquarium dosing writes them

Methods and test kits use the ions, labels use elements or oxides, and the factors between them are fixed by atomic weights ^[11]:

• Nitrogen: dosed as KNO3 and reported as nitrate. NO3 = N × 4.427, so 10 mg/L of nitrate is 2.26 mg/L of nitrogen.
• Phosphorus: dosed as KH2PO4 and reported as phosphate. PO4 = P × 3.066.
• Potassium: reported as K. Every common N and P salt carries potassium with it. Seachem makes the same point about its own range: potassium from nitrate or phosphate salts always brings N or P along, so its potassium product is made from potassium sulfate to raise K alone ^[7].

Seachem’s Flourish Nitrogen is a useful reminder that products can mix forms: it declares 1.5 % total nitrogen derived from potassium nitrate and urea ^[6], so part of its nitrogen is not nitrate.

How much do plants use?

Estimates come from the dosing methods’ authors, not from controlled studies, and they differ by design:

• EI’s author lists typical uptake at high light and CO2 as 1–5 ppm of nitrate and 0.5 ppm of phosphate per day, and doses a surplus above that ^[8].
• The PPS author estimates daily uptake at about 1 ppm of nitrate plus fish waste, and warns that nitrate should never drop to zero in a fully planted tank, because recovery from stunted growth is slow ^[9].

Tank light, CO2 and plant mass explain most of the gap. The EI and PPS-Pro guides show how each method turns its estimate into a dose.

Ammonium versus nitrate

Plants can take nitrogen up as either nitrate or ammonium ^[1]. EI’s author goes further, arguing from his own trials that ammonium, even at low levels, is the primary nutrient trigger for algae blooms in CO2-enriched tanks, while nitrate added as KNO3 is not ^[8]. This site reports that as the method author’s claim. In practice it is one reason dry-salt methods use potassium nitrate rather than ammonium salts.

Bottled fertilizers with and without N and P

Tropica sells two liquid fertilizers that differ mainly in N and P. Premium Nutrition declares 0.0 % nitrogen and 0.0 % phosphorus, with potassium, magnesium, sulfur, iron and trace elements, and is aimed at tanks with few or slow-growing plants and many fish ^[4]. Specialised Nutrition adds 1.3 % N and 0.1 % P for tanks with many fast-growing plants, with the advice to halve the dose and change water more often if algae appear ^[5]. Tropica pairs the nitrogen-free product with tanks holding many fish, and the PPS author likewise lists fish waste among the sources of nitrogen and phosphorus ^[10].

Potassium in practice

Potassium deficiency has no immediate visible symptom at first, only slower growth; later, older leaves develop chlorosis and burned margins, sometimes with small white or yellow spots along the edges ^[2]. Seachem’s directions for its potassium product list the signs it associates with aquarium plants: yellowing and browning of older leaves, pinholes, and weak stems and roots ^[7]. Because KNO3 and KH2PO4 already add potassium, the EI planner computes K from all three salts before adding K2SO4.