Planted Aquarium EI Fertilizer Dosing
The Estimative Index (EI) Method for Planted Aquariums
Planted aquariums are living ecosystems where nutrient balance directly impacts plant growth and water quality. Unlike traditional aquariums with fish only, planted tanks must provide macronutrients (nitrogen, phosphorus, potassium) and micronutrients (iron, boron, manganese, zinc, copper, molybdenum) that fish waste alone often cannot supply in sufficient quantities. The Estimative Index (EI) method, developed by aquatic plant researcher Florian Käser (also known as Estimative Index), provides a systematic approach to fertilizer dosing by maintaining target nutrient concentrations regardless of actual depletion rates. Rather than measuring nutrients (which is expensive and impractical for aquarists), the EI method assumes plants will consume nutrients proportional to growth and light availability, and compensates with fixed dosing schedules followed by regular water changes to prevent accumulation.
The EI method's elegance lies in its simplicity and effectiveness: dose fixed amounts of macronutrients daily (or three times weekly), perform a 50% water change weekly, and observe plant health. If plants look vibrant with good coloration and growth, dosing is optimal. If algae blooms, reduce macronutrient dosing; if plants show deficiencies, increase dosing. This feedback loop is far more practical than constant testing and allows aquarists of any experience level to maintain thriving planted systems.
EI Macro and Micro Nutrient Formula
The standard EI target is to increase nutrient levels by a specific amount weekly, then remove half via water change, and repeat. The weekly targets are:
This is equivalent to increasing concentrations by 10 ppm nitrogen, 2 ppm phosphorus, and 20 ppm potassium across the tank volume each week. Since plants consume nutrients and a 50% water change removes half the dissolved nutrients, maintaining these targets requires dosing slightly more than the consumption rate. Daily dosing is calculated as:
where is tank volume in liters and is a concentration conversion factor (typically 1/600 for milligrams to achieve ppm in aquariums). Dividing weekly targets by 6 accounts for 6 doses in a week plus margin for variations in consumption.
Worked Example: Dosing a 100-Liter Planted Tank
An aquarist maintains a 100-liter planted tank with 8 hours of daily light and 50% weekly water changes. Using the standard EI method, calculate daily macro dosing:
Step 1: Determine tank volume – 100 liters.
Step 2: Calculate nitrogen dose – Standard EI weekly target: 10 ppm. Assuming potassium nitrate (KNO₃) at 13.7% nitrogen: dose required = 10 ppm × 100 L ÷ 1000 × (100 ÷ 13.7) ≈ 73 grams per week. Daily dose (dividing by 6–7 days): ~11 grams per day.
Step 3: Calculate phosphorus dose – Weekly target: 2 ppm. Using potassium phosphate dibasic (K₂HPO₄) at 18% phosphorus: 2 ppm × 100 L ÷ 1000 × (100 ÷ 18) ≈ 11 grams per week, or ~1.6 grams daily.
Step 4: Calculate potassium dose – Weekly target: 20 ppm. Using potassium sulfate (K₂SO₄) at 44% potassium: 20 ppm × 100 L ÷ 1000 × (100 ÷ 44) ≈ 45 grams per week, or ~6.5 grams daily.
Step 5: Micro dose – Standard EI micro (trace elements) dose: 2 drops per gallon (~0.5 mL per 10 L) of a comprehensive trace element solution (like Flourish Excel or equivalent) three times weekly.
Result:** Daily dosing protocol: 11g KNO₃, 1.6g K₂HPO₄, 6.5g K₂SO₄, plus trace element solution three times weekly. After one week of dosing, perform a 50% water change and repeat.
EI Dosing Schedule and Frequency Variations
The following table shows typical EI target adjustments for different tank conditions:
| Tank Condition | Weekly N Target (ppm) | Weekly P Target (ppm) | Weekly K Target (ppm) | Notes |
|---|---|---|---|---|
| Low light, slow growth | 5 | 1 | 10 | Reduce to prevent accumulation in slow-growing tanks |
| Standard EI (moderate growth) | 10 | 2 | 20 | Most planted tanks; most common dosing |
| High light, fast growth | 15 | 3 | 30 | Increase for high-growth tanks or competition algae pressures |
| Very high light (45+ W/100L) | 20 | 4 | 40 | Only for competitive high-growth systems; risk of algae if not pruned aggressively |
Light intensity is the strongest predictor of plant growth rate and nutrient demand. Tanks with intense lighting (45+ watts per 100 liters) grow plants vigorously and consume nutrients faster; thus, higher EI dosing prevents deficiencies. Conversely, dim tanks with slow-growing plants (low-light crypts and Anubias) may accumulate excess nutrients, promoting algae; lower dosing is prudent.
Micro (Trace Element) Dosing and Iron
Beyond macronutrients, plants require iron, manganese, boron, and other trace elements. The EI method typically addresses micros with a comprehensive trace element solution dosed three times weekly. Iron (Fe) is particularly important; many aquarium plants show yellow leaves with green veins (chlorosis) when iron-deficient. Standard trace solution dosing is 2 drops per gallon (approximately 0.5 mL per 10 liters) three times per week. For high-light, high-growth tanks, some aquarists increase to daily trace dosing. Iron concentration in quality trace solutions is typically 1 mg/L, so 0.5 mL per 10 L adds approximately 0.05 ppm Fe per dose—a safe and effective level.
Water Change Management and Nutrient Accumulation
The 50% weekly water change is integral to EI. Without it, nutrients accumulate to toxic levels, and the system can shift from nutrient-limited (good plant growth) to nutrient-saturated (algae dominates). The water change removes excess nutrients, resets the system, and prevents osmotic imbalance from salt accumulation. Aquarists who skip or reduce water changes often see algae blooms; those who increase water change frequency (75% weekly) can dose more aggressively. The relationship is direct: dosing must be calibrated to water change frequency.
Using the Calculator
Enter your tank volume in liters. Set your weekly water change percentage (50% is standard; 25–75% is typical). Enter daily light hours (used to estimate plant growth rate). Select an EI dosing target (standard is most common; high for fast-growth competitive tanks; low for slow growth). The calculator computes the grams per day of each macronutrient fertilizer (KNO₃, K₂HPO₄, K₂SO₄) and recommends trace element dosing frequency. Purchase fertilizers separately or use all-in-one formulations that bundle all three macros; the calculator helps you determine the correct daily amount regardless of product choice.
Fertilizer Choice and Product Options
Aquarists can buy fertilizer components (pure salts) individually and measure/mix daily, or purchase pre-formulated all-in-one products that combine macro and micro in the correct ratios. Component dosing is cheaper long-term but requires a milligram scale (~$10–20). All-in-one products (like Flourish Comprehensive, APT, or Thrive) are more expensive per dose but convenient and eliminate measurement error. The calculator works with either approach; daily dosages calculated here can be scaled to product-specific instructions.
Limitations and Troubleshooting
EI assumes plants and algae consume nutrients proportionally and that the system has sufficient light and CO₂ for plant growth. If CO₂ is limiting (below 20 ppm in the tank), plants grow slowly and don't consume proportional nutrients, leading to nutrient accumulation and algae. Similarly, if lighting is insufficient, plants stagnate and algae exploit the nutrient surplus. The calculator provides dosing; success depends on overall tank management: lighting, CO₂ injection, pruning, substrate nutrients, and water quality. If algae blooms despite proper dosing, investigate light and CO₂ first; nutrient excess is usually a symptom, not the root cause. Additionally, tap water nutrients (nitrogen and phosphorus from municipal supplies) vary; soft-water aquarists starting with zero baseline may need full EI dosing, while hard-water systems may need less.