Fertilizing a planted aquarium: macros, micros and target levels

Macronutrients and micronutrients

• nitrogen (N) – as nitrate NO₃, the base of growth,
• phosphorus (P) – as phosphate PO₄, energy and roots,
• potassium (K) – water regulation in tissues,
• micronutrients – iron (Fe), manganese, boron, zinc and others in small doses.

Plants get magnesium and calcium from water hardness (GH). In very soft water or RO they have to be added by remineralizing.

Guideline levels

• NO₃: 10–20 mg/l
• PO₄: 0.5–2 mg/l
• K: 10–20 mg/l
• Fe: 0.05–0.1 mg/l

These are sensible starting numbers for a planted tank, not exact targets. What matters more is that the levels stay stable and match how the plants grow.

Methods: EI, lean dosing and all-in-one

The Estimative Index (EI) method deliberately doses more than plants use, and the surplus is removed with a 50 % water change once a week. It is simple and plants never starve, but it needs regular changes and stable CO₂.

Lean dosing adds only what plants consume and is guided by testing. Fewer water changes, more testing.

All-in-one fertilizers contain macros and micros in one ratio. For most tanks without extreme demands they are the simplest route.

Aquasoil and root tabs

Aquasoil supplies nutrients for the first months, so weaker water-column dosing is enough at the start. Root feeders – cryptocorynes, swords, carpeting plants – benefit from root tabs after a few months.

Spotting a deficiency

• whole older leaves turning yellow – often nitrogen,
• small pinholes in older leaves – potassium,
• pale to white new growth with green veins – iron or other micros,
• green spots on glass and leaves, slow growth – often too little phosphate,
• stunted, twisted tips – calcium or boron.

Lack of CO₂ or light can cause the same symptoms. Check CO₂ before raising doses.

Consistency and records

Plants and algae respond to changes with a delay. When you log doses and measurements you can see the link between what you changed and what happened in the tank a week or two later.