Estimative Index (EI) dosing: what the method prescribes

The idea in two sentences

The Estimative Index, as Tom Barr described it in 2005, is a way to dose a planted tank without test kits: “the aquarist doses frequently to prevent anything from running out” and “does large weekly water changes to prevent any build up” ^[1]. The result is a nutrient level that stays inside a known range, estimated from the dose rather than measured.

Barr’s argument is that plants need a non-limiting supply more than a precise residual level, and that the range of concentrations plants tolerate above deficiency is wide ^[1]. Instead of tuning each nutrient to a target, EI overshoots on purpose and uses the water change as the safety valve.

Why a 50 % water change caps the build-up

The arithmetic is the core of the method. If you add D mg/L of a nutrient every week and replace a fraction f of the water once a week, the level settles where each change removes exactly what the week added. Just before each change the level is D ÷ f, assuming no uptake at all. With f = 0.5 that is 2 × D, which is the figure Barr gives: with a general 50 % weekly water change, a nutrient builds up to “a maximum of 2x” the weekly dose ^[1].

EI Light works the same arithmetic for other change sizes: 10 ppm of nitrate a week peaks at 20 ppm with 50 % weekly changes, about 12 ppm with 90 %, and about 40 ppm with 25 % ^[2]. The water-change calculator draws this curve for any dose and change size.

Real tanks sit below these ceilings because plants take nutrients up. Barr lists typical uptake at high light and CO2 as 1–5 ppm of nitrate and 0.5 ppm of phosphate per day ^[1], which is why EI’s doses look large next to the ceiling arithmetic.

What EI prescribes

Target ranges

Barr’s original article and the later EI Light write-up give slightly different ranges ^{[1, 2]}:

	Original article	EI Light
CO2	25–35 mg/L	25–35 mg/L
NO3	5–30 mg/L	10–30 mg/L
K	10–30 mg/L	10–30 mg/L
PO4	1.0–3.0 mg/L	1.0–2.0 mg/L
Fe	0.2–0.5 mg/L “or higher”	0.2–0.5 mg/L “or higher”
GH	about 3 degrees (~50 ppm) or higher	1–2 degrees “extra” (17–40 ppm)

Dosing schedule

EI Light gives a “General Dosing Guideline for High Light and well planted aquariums” in teaspoons, three times a week, with a 50 % weekly change ^[2]. For a 20–40 US gallon tank, for example: about 1/4 teaspoon of KNO3, 1/16 teaspoon of KH2PO4 and 1/16 teaspoon (5 mL) of trace elements three times a week, and 1/2 teaspoon of GH booster once a week after the water change. The same table runs from 10–20 gallon to 100–125 gallon tanks, and every amount is marked “+/-”.

EI Light gives no gram equivalents, and the original article gives only one, ¼ teaspoon of KNO3 as 1.67 g ^[1]; a teaspoon of a crystalline salt has no fixed weight. To dose EI by weight, start from the target ranges and the ceiling arithmetic instead. The EI and PPS-Pro planner does that: you choose the highest level you accept before the water change, and it returns grams of KNO3, KH2PO4, K2SO4 and trace mix per dose. A 20 mg/L nitrate ceiling at 50 % gives 10 mg/L a week, Barr’s own example ^[1]; in a 75 L tank dosed three times a week, that is 0.41 g of potassium nitrate per dose, using its molar mass of 101.10 g/mol ^[4].

Trace elements

EI Light treats Plantex CSM+B, Seachem Flourish and Tropica’s trace product as interchangeable trace sources ^[2]. The manufacturer’s own analysis for the base Plantex NutriTrace CSM lists 7 % iron, 2 % manganese, 0.4 % zinc, 0.1 % copper, 0.06 % molybdenum and 1.5 % magnesium, with no boron ^[3]. The “CSM+B” named in EI Light contains boron by name, so its exact percentages depend on the blend you buy; read the bag.

What EI does not claim

The write-ups are explicit about their limits:

• It is not a fixed recipe. EI Light says the scheme is not meant to be “written in stone” and that the aquarist should adjust it ^[2]. Barr notes tanks with heavy fish loads or nutrient-rich tap water may need less ^[1].
• It assumes good CO2. Barr suggests about 30 mg/L of CO2 and says that no nutrient routine will perform well without good, stable CO2 ^[1]. EI Light adds that most issues revolve around CO2 and water flow when nutrients are not limiting ^[2].
• It is built for high light. Low-light tanks can dose less and less often, once or twice a week ^[2].

Barr also argues that excess nitrate, phosphate and iron do not trigger algae in a healthy, CO2-enriched planted tank, and that ammonium is the main nutrient-related trigger ^[1]. That is a claim from his own trials; the write-ups present it as the author’s observation, not as a controlled study, and this site reports it the same way.

EI and PPS-Pro side by side

EI floods and resets; PPS-Pro doses small daily amounts close to estimated uptake and aims at a lean water column. The two use the same salts. The planner computes both from their authors’ published numbers so you can compare the weekly totals for your own tank.