AquariumCalcs: Fin & Figures — for the fish aficionado
Meet the Calcs Family →

Fertilizer Dosing Calculator (EI Method)

The Estimative Index (EI) method doses on a schedule and resets any excess with a large weekly water change, rather than trying to precisely match consumption. These are commonly used base ratios, scaled to your tank size.

Your numbers
KNO3 (nitrate)
KH2PO4 (phosphate)
K2SO4 (potassium)
Trace mix (micros)

Green algae means dial back fertilizer, not add light

Watch for a spike in green algae as an early warning sign of dosing out of balance with light and CO2 — dial back the fertilizer rather than adding more light to compensate.

What the Estimative Index method actually is

EI, developed by Tom Barr, deliberately doses fertilizer somewhat in excess of what plants are expected to consume in a week, then resets any accumulation with a large (commonly 50%) weekly water change. This avoids the need for precise weekly water testing to hit an exact target — the water change is what keeps things from building up, which is the defining feature of the method, not the specific dose amounts.

How these doses are scaled

The ratios above (KNO3, KH2PO4, K2SO4 and a trace/micro mix, about 1/8 teaspoon of KNO3 and about 1/32 teaspoon each of KH2PO4, K2SO4 and the trace mix per 10 gallons, dosed three times a week) are commonly published EI starting ratios, scaled here to your tank's volume. Actual dry fertilizer potency varies somewhat by brand and hydration state, so treat this as a solid starting point to dial in against your own plants' growth and any algae response, not an exact prescription.

Dosing schedule

A common weekly pattern alternates macro-nutrient days (KNO3, KH2PO4, K2SO4) with micro-nutrient/trace-mix days, three times each per week, with the water change on a seventh "rest" day. Heavily planted, fast-growing tanks with strong lighting and CO2 injection benefit most from full-strength EI dosing; lightly planted or low-light tanks often do better started at half this dose.

Source: Dosing ratios reflect the Estimative Index (EI) method developed by Tom Barr, a widely referenced dry-fertilizer dosing approach in the planted-tank hobby.

Frequently asked questions

Do I need to test nitrate and phosphate weekly with EI dosing?

Not routinely, which is the point of the method — the weekly water change is what prevents buildup. Testing occasionally to make sure levels aren't climbing unexpectedly, especially early on, is still a good habit.

Is EI dosing safe for fish?

Yes, when combined with its required weekly water change. The doses are calibrated together with that water change as a system, so skipping the water change for an extended period is where problems (algae, nutrient buildup) tend to appear.

Can I use liquid fertilizer instead of dry powders with EI?

EI is most commonly associated with dry fertilizer salts because they're inexpensive at the doses involved, but the same underlying macro/micro ratios and weekly-reset logic can be adapted to liquid ferts if you calculate the equivalent elemental dose from the product label.

More Planted Tank calculators

Aquascape Hardscape Ratio Calculator CO2 Injection Rate Calculator

All AquariumCalcs tools