EI Dosing Method: Liquid Fertilizer Schedule for Planted Tanks

The Estimative Index, or EI, is a liquid fertilizer dosing method for planted aquariums developed by aquatic-plant researcher Tom Barr. Instead of precisely matching fertilizer to what plants actually consume, EI deliberately doses nutrients in excess of what the plants can use, then relies on a large weekly water change to reset the tank and prevent that excess from building up.

Key Takeaways

• EI dosing intentionally overshoots nutrient demand rather than trying to precisely track it, which removes the need for regular test-kit monitoring of macronutrients.
• The method doses nitrogen, phosphorus, and potassium as "macros" and iron plus trace elements as "micros," typically on alternating days across the week.
• A large weekly water change, commonly around 50 percent, is not optional under EI - it's the mechanism that resets nutrient concentrations and prevents runaway buildup.
• EI is built for higher-light, CO2-injected planted tanks, where plants grow fast enough to actually use the nutrient surplus; low-tech, no-CO2 tanks need a much lighter dosing approach instead.
• The most common failure mode isn't the heavy dosing itself, it's skipping the weekly water change, which lets nutrients and dissolved solids accumulate and often shows up as an algae problem.

What Is the Estimative Index (EI) Dosing Method?

EI is a fertilizing method for planted tanks, popularized through the writing and forum posts of Tom Barr, that treats "close enough but generous" as the goal rather than precisely matching what the plants take up. Rather than testing water for nitrate, phosphate, or potassium and dosing to a specific target, an EI schedule simply adds a known, deliberately generous amount of fertilizer on a fixed schedule, trusting that any true excess gets removed by the water change at the end of the week. The original Estimative Index discussion Tom Barr posted on the Barr Report forum frames this explicitly as a way to stop needing test kits at all for routine dosing, since the method is designed to never let a nutrient run low enough to limit growth in the first place.

What Nutrients Does EI Dosing Involve?

Macronutrients

The three macronutrients dosed under EI are nitrogen (as nitrate, NO3), phosphorus (as phosphate, PO4), and potassium (K). These are the nutrients plants need in the largest quantities, and they're the ones most directly tied to visible growth rate.

Micronutrients

The micronutrients are iron and a blend of trace elements (commonly including manganese, boron, zinc, copper, and molybdenum, generally supplied as a chelated all-in-one "micro" mix). Iron in particular is closely watched under EI dosing because iron deficiency shows up visibly as pale new growth, and because activated carbon can strip chelated iron and other trace elements out of the water column if it's run continuously in a heavily dosed planted tank, which is one reason carbon use tends to be more selective in an EI setup than in a typical community tank.

What Does a Typical EI Dosing Schedule Look Like?

EI is generally organized around alternating macro and micro dosing days across the week, with one day reserved for the water change and no fertilizer dosing at all:

Exact per-dose amounts scale with tank volume, light intensity, and CO2 injection, and hobbyists typically use a fertilizer calculator or a pre-mixed EI product to work out doses for their specific tank, rather than fixed universal numbers.

Why Is the Weekly Water Change Non-Negotiable?

The water change is not a cleanup step tacked onto EI dosing, it's the actual reset mechanism the whole method depends on. Because EI deliberately doses more nutrients than the plants will use in a given week, those un-absorbed nutrients and accumulating dissolved solids need somewhere to go; a large water change is what removes them before the next week's dosing starts stacking on top of what's still there. Skip the water change, and the tank doesn't just have "a bit more fertilizer than needed," it can accumulate nutrient levels and total dissolved solids well beyond what EI was ever designed to run at, which is a very different situation from the method working as intended.

Who Should Use EI Dosing (and Who Shouldn't)?

EI is built for higher-light, CO2-injected planted tanks, since fast, CO2-fueled growth is what actually consumes the nutrient surplus EI provides. In a tank like that, pressurized or DIY CO2 injection removes the carbon limitation on growth, letting plants take up the heavy nutrient dosing productively instead of just leaving it in the water column for algae to use. Lower-light tanks running without CO2 injection are generally better served by a much lighter dosing approach scaled to their slower growth rate; our guide to setting up a planted aquarium without CO2 covers that lighter, low-tech alternative in detail.

Common EI Dosing Mistakes

• Skipping or shortening the weekly water change. This is the single most common way EI dosing goes wrong, and it's usually what's behind an algae outbreak that gets blamed on "too much fertilizer" when the real issue is a missed reset.
• Dosing EI-level amounts in a low-light or no-CO2 tank. Without strong growth to use the surplus, those nutrients just sit in the water column, which favors algae rather than plants.
• Running activated carbon continuously alongside heavy micro dosing, which can quietly strip iron and other trace elements the plants are relying on.
• Not keeping an eye on nitrate accumulation between water changes, especially in a heavily stocked or heavily fertilized tank; a guide to lowering nitrates in an aquarium is a useful reference if levels are creeping up faster than the weekly water change resets them.