What is EI dosing?

The Estimative Index is a way to fertilize a planted tank without test kits: add a little more of each nutrient than fast-growing plants can use, then reset the water with a big weekly change.

The idea in one paragraph

Tom Barr described EI in the mid-2000s for high-light, CO₂-injected tanks. Rather than chasing exact readings, you dose nitrogen, phosphorus, potassium and trace elements on a fixed schedule at rates set above the measured maximum uptake of vigorous stem plants. Because nothing runs out, plants are never limited by nutrients. A large water change every week stops anything from building up indefinitely.

Why the water change is the "index"

If you add the same amount every week and remove a fixed fraction of water, the level stops climbing and levels off. With a 50 % weekly change, the highest it can reach just before the change is twice what you add per week. That is why EI can skip test kits: the math, not a reading, tells you the ceiling. Our calculator shows this as "builds to" for every element.

Rule of thumb: level just before a change ≈ weekly dose ÷ water-change fraction. 20 ppm NO₃ a week with 50 % changes settles at about 40 ppm.

Typical EI numbers

Common EI guidance works out to roughly these amounts added per week, usually split into three doses:

Nutrient	Per week	Main salt
Nitrate (NO₃)	20–30 ppm	KNO₃
Phosphate (PO₄)	2–5 ppm (often 3)	KH₂PO₄
Potassium (K)	20–40 ppm	KNO₃ + K₂SO₄
Iron (Fe) via trace mix	≈0.5 ppm	CSM+B
Magnesium	only if tap water is soft	MgSO₄·7H₂O

The classic schedule: macros on Monday, Wednesday and Friday, micros on Tuesday, Thursday and Saturday, water change on Sunday. Keeping macros and micros on different days avoids phosphate and iron meeting at high concentration.

When EI is not the right fit

No CO₂, low light: plants grow slowly and use far less. Full EI just feeds algae. Try low-tech dosing or lean EI.
You can't do big water changes: without the reset, levels keep rising. Use a leaner, uptake-matched method like PPS-Pro.
Sensitive livestock such as some shrimp: keep traces modest and watch copper.

Work out your doses

Open the calculator with EI preloaded for a 20 US gal, 40 US gal or 200 L tank. Switch to lean EI if your light is moderate.

FAQ

How many times a week do I dose EI?

Usually macros three times and micros three times on alternate days, with a 50 % water change once a week. Low-light tanks can dose once or twice.

Do I need test kits for EI?

Not to stay in a safe range, because the weekly water change caps the build-up. Tests are still useful when troubleshooting.

Where to buy · Retailer links coming soon.

Open the dosing calculator More guides

Sources

Written in our own words from the sources above. Numbers are educational starting points; check product labels and your water.