The EI Philosophy: Surplus Over Precision
Traditional planted tank fertilizing tried to calculate exact plant nutrient uptake and dose precisely — a strategy that fails in practice because plant growth rates vary daily with light, CO2, temperature, and species. Tom Barr's Estimative Index inverts this: dose enough of every nutrient that plants are never limited by any deficiency, accept that some nutrients will accumulate during the week, and reset with a 50% water change every 7 days. Simplicity and reliability are the design goals, not precision.
EI works because most nutrients are not toxic to fish at the concentrations maintained by the dosing schedule. Potassium at 30 ppm has no measurable effect on fish. Nitrate at 20 ppm is safe for most species. Phosphate at 2 ppm is non-toxic. Only copper (part of micronutrient mixes) needs attention with invertebrates — invertebrate-safe micro formulas (EDTA-chelated copper) are essential in shrimp tanks.
The one genuine limitation of EI is algae risk in low-light or no-CO2 tanks. EI was designed for high-tech setups: strong light (40+ PAR at substrate) and CO2 injection at 25–30 ppm. In low-tech tanks, high phosphate doses can fuel algae without the fast plant growth to consume it. Modified lean-EI (50% of standard doses) is used in low-tech situations.
Macro Nutrients: NPK Formulas and Weekly Schedule
EI macros consist of three elements: Nitrogen (N) dosed as Potassium Nitrate (KNO3), Phosphorus (P) dosed as Monopotassium Phosphate (KH2PO4), and Potassium (K) sourced from both KNO3 and KH2PO4 with additional Potassium Sulfate (K2SO4) if needed. For a standard 100-liter planted tank with moderate-to-high plant density, target weekly dosing: NO3 10–20 ppm, PO4 1–2 ppm, K 10–30 ppm.
The standard EI schedule runs 3 days on macros, alternating with 3 days on micros, with a 50% water change on day 7. Example schedule: Monday/Wednesday/Friday = macro dose; Tuesday/Thursday/Saturday = micro dose; Sunday = 50% water change + restart. This alternating schedule prevents calcium precipitation reactions between macros and some chelated micro formulas that can occur if dosed simultaneously.
Pre-mixed macro stock solution for convenience (enough for 30 doses into a 100-liter tank): dissolve 28g KNO3 + 5g KH2PO4 + 15g K2SO4 in 500 ml distilled water. Dose 16 ml of this solution per 100 liters, three times per week. This delivers approximately NO3 +5 ppm, PO4 +0.5 ppm, K +6 ppm per dose — weekly total of +15 ppm NO3, +1.5 ppm PO4, +18 ppm K before the Sunday reset.
- ✦Weigh dry salts with a 0.01g digital scale — 1g error in KH2PO4 represents 20% of the weekly phosphate budget
- ✦Store stock solutions in dark glass bottles at room temperature for up to 6 months
- ✦If NO3 accumulates above 40 ppm by Saturday, reduce KNO3 in the stock by 20% or add an extra mid-week water change
Micronutrients: Iron, Manganese, Boron, and Trace Elements
Micro dosing in EI covers iron (Fe), manganese (Mn), boron (B), zinc (Zn), molybdenum (Mo), and copper (Cu). Iron is the most consumed micronutrient in planted tanks and the most commonly deficient — new growth showing pale yellow-green while old leaves remain green is a classic iron deficiency symptom. Target Fe concentration in the water column: 0.1–0.5 ppm (measured immediately after dosing; chelated iron degrades quickly in bright light).
Pre-mixed CSM+B (Compound Specific Micro + Boron) is the most widely used EI micro formula: 7g CSM+B dissolved in 250 ml distilled water, dosed at 5–10 ml per 100 liters three times per week on alternating days from macros. CSM+B contains: Fe 7.0%, Mn 2.0%, Zn 0.4%, Cu 0.1%, B 1.3%, Mo 0.06% in DTPA or EDTA chelation. DTPA chelation is superior for tanks with pH above 7.0 because it remains soluble to pH 7.5; EDTA chelation releases iron primarily at pH 5.5–7.0.
Aquascapers using CO2-injected tanks with ADA Aquasoil (pH 6.2–6.8) can use either chelation form successfully. Tanks running at pH above 7.0 (hard water, limestone substrate) should specifically source DTPA-chelated iron mixes and check that any commercial micro fertilizer specifies DTPA, not EDTA alone.
- ✦Iron deficiency (interveinal chlorosis on new leaves) develops within 5–7 days of missed micro dosing in fast-growing tanks
- ✦Overdosing copper above 0.15 ppm kills shrimp within 24 hours — use invertebrate-safe micro blends that exclude copper
- ✦Test iron concentration 2 hours after micro dose to measure peak; do not test immediately after dosing (too high) or the next day (too degraded)
Diagnosing Nutrient Deficiencies in EI Tanks
Even with correct EI dosing, deficiencies appear when plant uptake temporarily spikes or a parameter (CO2, light, temperature) changes. Key visual diagnostic chart: Yellow-green NEW leaves = iron or manganese deficiency (dose micros immediately). Yellow OLD leaves with green veins = potassium deficiency (increase K2SO4 in macro mix). Holes in old leaves or necrotic edges = phosphate deficiency (dose KH2PO4 immediately). Purple-red tinge on leaf undersides of green species = phosphate deficiency or cold stress. Completely pale new growth = severe multi-element deficiency or CO2 crash.
A CO2 crash (CO2 dropping below 10 ppm) mimics potassium deficiency because low CO2 shuts down the carbon fixation process that drives nutrient transport through the plant. Before adjusting EI dosing, always verify CO2 is at 25–30 ppm using a drop checker with 4 dKH reference solution (green = correct, yellow = too high, blue = deficient).
EI deficiency recovery timeline: iron deficiency resolves visually in 5–7 days after correction. Phosphate deficiency resolves in 3–5 days (phosphate is rapidly mobile in plant tissue). Potassium deficiency is slow to resolve — 10–14 days of correct K dosing before clear new growth appears, because K is slowly mobile in plant tissue.
Calibrating EI for Tank Size and Plant Density
Standard EI doses are calibrated for a heavily planted tank (80%+ substrate coverage, medium to fast-growing species). A lightly planted tank or one dominated by slow-growing species like Anubias and Java fern needs 30–50% of standard EI doses — at full EI doses, nutrients accumulate faster than plants can use them and trigger algae. A Dutch style tank at maximum density may actually need 120–130% of standard doses in week 2–3 of a growth surge after major trimming.
Nano tanks under 30 liters require extra care with EI because small water volumes have higher concentration sensitivity — 1g of KNO3 raises NO3 by 5 ppm in 100 liters but by 17 ppm in 30 liters. Use a 10× diluted stock solution for nano tanks and dose by the milliliter (3–5 ml per dose) rather than by measuring dry salts directly into the tank. Precision matters more as tank size decreases.
EI is not the only valid approach — PPS-Pro (Perpetual Preservation System) and ADA's Brighty series both produce excellent planted tanks. PPS-Pro targets strict non-accumulating doses (all nutrients consumed daily, no weekly reset needed) and works well for the hobbyist who forgets water changes. ADA Brighty Step 1/2/3 are commercial formulations with undisclosed ratios designed for ADA Aquasoil systems. EI's advantage is complete transparency: you know exactly what is in the water and can adjust each element independently based on what your plants tell you.
- ✦Run a Seachem test kit baseline on tap water before building EI doses — tap potassium, phosphate, and nitrate already present reduce how much you need to add
- ✦After any water change larger than 60%, redose the full weekly macro amount immediately to avoid a crash-dose shock
- ✦Keep a spreadsheet log of weekly test results alongside doses — nutrient trends over 8 weeks reveal your tank's true consumption rates