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 roughly 8-10 ppm of NO3 per dose, which is textbook EI. For KH2PO4 (about 70% phosphate), grams = ppm × liters ÷ 698, and the base dose lands near 1.4 ppm of PO4.

Lighting also affects uptake. High light (above 50 PAR, photosynthetically active radiation, at the substrate) more than doubles nutrient demand compared to low light (below 20 PAR); the multipliers span 0.6× to 1.5×.

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.