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Aquarium Fertilizer Calculator — EI Dry Dosing for Planted Tanks

Calculate weekly dry fertilizer doses (KNO3, KH2PO4, K2SO4, CSM+B micros) for your planted aquarium using the proven Estimative Index method. Works for high-tech CO2 and low-tech non-CO2 setups.

Calculator

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EI Dry Dosing — per dose, 3× weekly
  • KNO3 (potassium nitrate): 0.375 tsp
  • KH2PO4 (mono-potassium phosphate): 0.094 tsp
  • K2SO4 (potassium sulfate): 0.188 tsp
  • CSM+B trace (micros): 0.094 tsp

EI Protocol

  • Dose macros (KNO3, KH2PO4, K2SO4) on Mon/Wed/Fri
  • Dose micros (trace) on Tue/Thu/Sat
  • Water change: 50% weekly to reset
EI (Estimative Index) by Tom Barr provides excess nutrients; weekly water changes prevent buildup. For non-CO2 tanks, use 1/3 the doses.

How to use the fertilizer calculator

  1. Enter tank volume — Use real water volume (tank volume minus substrate and decor).
  2. Pick tech level — High-tech (CO2 injection, 50+ PAR) uses full EI; low-tech uses 1/3 EI.
  3. Read weekly doses — The calculator returns KNO3, KH2PO4, K2SO4, and CSM+B in tsp per week.
  4. Split across 6 days — Alternate macros M/W/F and micros T/Th/Sa. 50 % water change Sunday.

The EI method — how it works

Before Tom Barr published the Estimative Index around 2001, planted-tank keepers micro-tested every nutrient (nitrate, phosphate, potassium, iron) constantly and tried to maintain exact concentrations. This was impractical: plant uptake varies hour to hour, test kits have ±20 % error, and different plant species have different preferences. EI sidestepped this by flipping the problem: dose generously, change water weekly, and never run out. The plants take what they need and the excess gets exported with the 50 % water change.

The insight behind EI is that nutrient excess (up to a point) isn't harmful to plants or fish, but nutrient scarcity causes plants to halt growth and cede carbon to algae. The weekly reset ensures nothing ever accumulates to toxic levels. Fifteen years of hobbyist experience confirms this works across scales from 5-gallon nanos to 500-gallon planted displays.

Standard EI targets per week

NutrientTargetSource salt
NO3 (nitrate)20 ppmKNO3 (potassium nitrate)
PO4 (phosphate)2 ppmKH2PO4 (monopotassium phosphate)
K (potassium)20 ppmK2SO4 + from KNO3
Fe + micros0.5 ppm FeCSM+B / Plantex
Mg (magnesium)10 ppmMgSO4 (Epsom) if needed

Diagnosing common planted-tank nutrient issues

Most “mysterious” planted-tank problems are nutrient deficiencies, not disease. Use visible symptoms to identify what's missing:

  • Nitrogen deficiency: New growth is pale yellow-green; old leaves yellow from the tip inward. Increase KNO3.
  • Phosphorus deficiency: Dark green leaves, green-spot algae on glass and older leaves. Increase KH2PO4.
  • Potassium deficiency: Pinholes in leaves (starting on older stem plants like rotala). Increase K2SO4.
  • Iron / micros deficiency: New growth is pale between green veins (interveinal chlorosis). Red plants turn green. Increase CSM+B micros.
  • Magnesium deficiency: Similar to iron but whole leaf yellows rather than interveinal. Add 1/8 tsp MgSO4 weekly.
  • CO2 deficiency (not a nutrient but commonly confused): Stunted new growth, compact short internodes, algae on new leaves. Check CO2 before adjusting ferts.

Frequently Asked Questions

What is EI (Estimative Index) fertilizer dosing?

Estimative Index (EI) is a dosing methodology developed by Tom Barr in the early 2000s that delivers intentionally excess macro and micronutrients to a planted aquarium, then resets any accumulation with a 50 % weekly water change. The benefit: plants never run short of any nutrient, and you don't need precise testing. EI is the de facto standard for high-tech CO2-injected tanks above 10 gallons.

Can I use liquid fertilizers instead of dry dosing?

Yes — Seachem Flourish, Thrive, NilocG Thrive+, and GLA's premixed liquids are all valid EI-equivalent products. They cost 10–20× more per year than dry dosing raw salts (KNO3, KH2PO4, K2SO4, CSM+B micros), but require zero mixing. For tanks under 20 gallons the cost difference is trivial ($30/year vs. $3/year). For 50+ gallon tanks, switching to dry salts can save $100+ annually.

Do non-CO2 (low-tech) tanks need ferts?

Yes, but at roughly 1/3 the EI dose because low-light, low-CO2 plant growth is much slower. Overdosing a non-CO2 tank triggers algae blooms because the plants can't keep up. A good starting point: dose macros twice weekly at 1/3 EI, micros once weekly, and water change 30–40 % weekly.

What's in the EI dosing schedule for a standard 40-gallon tank?

Monday/Wednesday/Friday: 1/4 tsp KNO3 + 1/16 tsp KH2PO4 + 1/8 tsp K2SO4 (macros). Tuesday/Thursday/Saturday: 1/16 tsp CSM+B (micros). Sunday: 50 % water change, reset. This delivers ~20 ppm NO3, 2 ppm PO4, 20 ppm K, and trace iron/manganese/etc. per week.

Do I need all four macros (N, P, K) + micros?

Yes. Missing any major macro (nitrogen, phosphorus, potassium) causes a specific deficiency: pale new growth (N deficiency), pinhole leaves (K), green-spot algae on glass (P starvation, not excess), dark veins on yellowed leaves (Mg). Micros (Fe, Mn, Zn, Cu, B, Mo) drive chlorophyll production and are required in trace amounts. Skipping micros turns red plants green.

Can I overdose plant fertilizer and hurt my fish?

Standard EI doses are well below fish toxicity thresholds. NO3 at 40 ppm is safe for nearly all freshwater fish; PO4 at 5 ppm is safe. The 50 % weekly water change prevents any buildup. The exception is copper in some micro blends — shrimp (especially neocaridina and caridina) are sensitive. Use shrimp-safe micros (Seachem Flourish or NilocG Thrive S) in shrimp tanks.

Should I dose macros and micros on the same day?

No. Alternate them — macros Mon/Wed/Fri, micros Tue/Thu/Sat. Mixing the two in a concentrated form causes iron precipitation (phosphate + iron = insoluble iron phosphate that settles out before plants can use it). In the tank itself the dilution is enough that this isn't a problem, but in dosing bottles it is.

Sources & References

  1. [1]
    The Estimative Index Dosing Guide — Barr Report (Tom Barr)
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