Aquarium Plant Fertilizer Dosing Calculator

How Aquarium Plant Fertilizer Dosing Works

Aquatic plants require three categories of nutrients: macronutrients (nitrogen, phosphorus, potassium), micronutrients (iron, manganese, boron, zinc), and carbon (typically supplied via CO2 injection). The Estimative Index (EI) dosing method provides a reliable baseline for planted tanks.

How This Calculator Doses

It follows the EI (Estimative Index) method: dose slightly more than the plants can use, then reset with a big weekly water change. The base rates, per 100 liters per dose (macros go in three times a week):

• Potassium Nitrate (KNO3): 1.6 g
• Monopotassium Phosphate (KH2PO4): 0.25 g
• Trace mix: 10 mL per 100 liters per week

Each rate is scaled by three multipliers: plant density (low 0.5×, medium 1.0×, high 1.5×), lighting (low 0.6×, medium 1.0×, high 1.5×), and CO2 (injected 1.3×, none 0.8×).

Chemistry sanity check: KNO3 is about 61% nitrate by mass, so the grams needed to raise NO3 by a target ppm (parts per million) = ppm × liters ÷ 613. The 1.6 g base dose in 100 L works out to 9.8 ppm of NO3 per dose, which is textbook EI. For KH2PO4 (about 70% phosphate), grams = ppm × liters ÷ 698, and the base dose lands at 1.7 ppm of PO4.

That is about 5.7 parts nitrate to 1 part phosphate by weight. Worth knowing if you have read about the Redfield ratio: converted to NO3 and PO4 by mass, Redfield is closer to 10 to 1, so EI deliberately doses roughly twice the phosphate that ratio implies. That is a design choice rather than an error. EI takes the view that a phosphate shortage causes more trouble than a phosphate surplus, so it errs on the generous side and lets the weekly water change clear the excess.

Lighting also affects uptake. High light (over 60 PAR, photosynthetically active radiation, measured at the substrate) more than doubles nutrient demand compared to low light (under 30 PAR); the multipliers span 0.6× to 1.5×. Those band edges match the PAR calculator and the CO2 injection calculator, so a PAR figure means the same thing on all three pages.

Worked Example

A 300-liter tank with high plant density, medium lighting, and pressurized CO2:

• Combined multiplier: 1.5 × 1.0 × 1.3 = 1.95
• KNO3: (300 ÷ 100) × 1.6 × 1.95 = 9.36 g per dose, three times a week
• KH2PO4: (300 ÷ 100) × 0.25 × 1.95 = 1.46 g per dose
• Trace mix: (300 ÷ 100) × 10 × 1.95 = 58.5 mL per week

EI Dosing Reference Table (medium density, medium light, with CO2)

Dose macros and micros on alternating days (Mon/Wed/Fri and Tue/Thu/Sat), with a 50% water change on Sunday to reset nutrient levels and prevent accumulation.

Keep them on separate days for a reason: iron in the trace mix and phosphate from KH2PO4 will precipitate out as iron phosphate if you pour them into the tank together. The cloud settles and neither nutrient is available to the plants.

EI without CO2 is the common way to get this wrong

The multipliers here will happily hand a high-light, heavily planted tank with no CO2 a bigger dose than a moderate tank running pressurised gas. Arithmetically that is what the numbers say, and in a tank it is a recipe for green spot algae and hair algae.
Light sets the pace, carbon caps it. If the plants cannot get the carbon to use the nitrate and phosphate you are pouring in, the surplus is available to something that can. On a tank without CO2, dial the light back first, and dose at somewhere between a quarter and a half of full EI. Chasing full EI without gas is how most people end up scrubbing glass every week.