Estimative Index (EI) Fertilizer Dosing for Planted Tanks

The Estimative Index (EI) doses nutrients in excess so plants are never limited, then resets the water column with a large weekly water change, removing the need for test kits.

Overview

The Estimative Index (EI) is a fertilizer-dosing method for planted aquariums coined by Tom Barr. Its central idea is to estimate generously rather than measure precisely: nutrients are added in excess so that no nutrient ever limits plant growth, and the water column is reset at the end of each week with a large water change so that nothing accumulates to harmful levels. Because the method floods the tank with nutrients on purpose, no test kits or exact uptake measurements are needed. EI is best suited to high-light tanks with added carbon dioxide, where plant demand is high.

The principle

Uptake rates are hard to measure because tanks contain many plant species growing under different light and CO2 conditions. EI sidesteps this by keeping every nutrient non-limiting, on the reasoning that vigorously growing plants outcompete algae. The actual drivers of growth are light and carbon dioxide; the fertilizer simply ensures nutrients are never the bottleneck. The weekly water change prevents the deliberately high doses from building up over time.

Typical weekly targets

• Nitrate (NO3): up to about 20-30 ppm cumulative per week
• Phosphate (PO4): up to about 5-7 ppm per week
• Potassium (K): up to about 20-30 ppm per week
• Iron and trace elements: up to about 0.5-1 ppm per week
• A roughly 50% water change at the end of the week to reset levels

Dosing schedule

Macronutrients (nitrogen, phosphorus, potassium) and micronutrient traces are typically dosed on alternating days, for example three times a week each. Macros and traces are usually added on separate days rather than mixed together, which avoids interactions between phosphate and iron. After the week's doses, a large water change of around half the tank volume returns the water to a low, known baseline so the next week starts fresh.

EI versus leaner approaches

EI deliberately runs high nutrient levels and relies on water changes to stay safe, which makes it simple and forgiving but water- and fertilizer-intensive. Leaner methods instead dose smaller, more targeted amounts to keep nutrients just sufficient, which can suit lower-light or low-tech tanks without CO2. The right choice depends on lighting, CO2 use, plant load, and how much maintenance the keeper wants to do.