Planted tanks · Aquarium Dosing Calculator
EI dosing schedule explained
Estimative Index (EI) dosing adds macronutrients three times a week and trace elements on the alternate days, then resets the tank with a 50 percent water change weekly. Weekly targets for a CO2 injected tank are roughly 20 ppm nitrate, 3 ppm phosphate, 20 to 30 ppm potassium, and 0.5 ppm iron, dosed in excess so plants never run short.
The idea behind Estimative Index
EI is a fertilization method developed by Tom Barr for CO2 enriched planted aquariums. Instead of testing water and topping up to a measured level, the aquarist estimates the most a healthy planted tank could consume in a week, doses slightly more than that, and relies on a large weekly water change to remove whatever the plants did not use. The name comes from that estimate.
Because the dosing is deliberately generous, nutrients are never the limiting factor. Light and CO2 set the growth rate, and the fertilizer schedule simply keeps up. The method assumes healthy plant mass, stable CO2 around 30 ppm during the photoperiod, and a real 50 percent water change every week. Take one of those away and the accumulation math stops working.
The published targets are rules of thumb, not fixed values. The CO2Art guide, which follows Barr's numbers, states plainly that they are general rules of thumb and not set in stone. Many tanks run well at half the standard amounts, especially with moderate light.
Weekly nutrient targets
For a high light, CO2 injected tank the weekly totals are approximately: nitrate (NO3) 20 ppm, phosphate (PO4) 3 ppm, potassium (K) 20 to 30 ppm, and iron (Fe) 0.5 ppm as a proxy for the full trace mix. Those totals are reached across three macro doses and three (or two) micro doses during the week.
Each macro dose therefore adds roughly one third of the weekly figure: about 7 ppm NO3, 1 ppm PO4, and 7 to 10 ppm K. Each micro dose adds about 0.1 to 0.2 ppm Fe. Sulfate from potassium sulfate and magnesium from Epsom salt come along as by products and are rarely limiting.
Low light tanks without CO2 use far less. A common approach is the same salts at about one quarter of the amounts, or one dose per week of each, since plant uptake without CO2 is a fraction of a high tech tank. Overdosing a low tech tank does not harm plants but wastes fertilizer and can push nitrate higher than livestock keepers want.
The seven day schedule
Day 1 (Monday): macros, meaning potassium nitrate (KNO3), monopotassium phosphate (KH2PO4), and potassium sulfate (K2SO4). Day 2 (Tuesday): micros, a trace mix such as CSM+B or a chelated trace product. Day 3: macros. Day 4: micros. Day 5: macros. Day 6: micros or rest. Day 7 (Sunday): 50 percent or larger water change, then start again.
Macros and micros are dosed on different days for a chemical reason. Phosphate and iron react in the water column to form insoluble iron phosphate. Dosing them at the same moment reduces how much of both stays available. Separating them by a day is enough; some aquarists dose both daily in a single small dose without visible problems, but the alternate day pattern is the standard EI form.
Dose before the lights come on so nutrients are available during the photoperiod, and dose into an area of flow so the dry salts or solution disperse quickly rather than settling on a leaf.
Dry dosing amounts by tank size
Green Leaf Aquariums publishes the standard EI dry dosing chart. Per dose (three times a week for macros, three for micros): 10 to 20 gallons: 1/8 tsp KNO3, 1/32 tsp KH2PO4, 1/32 tsp K2SO4, 1/32 tsp trace. 20 to 40 gallons: 1/4 tsp KNO3, 1/16 tsp KH2PO4, 1/16 tsp K2SO4, 1/16 tsp trace. 40 to 60 gallons: 1/2 tsp KNO3, 1/8 tsp KH2PO4, 1/8 tsp K2SO4, 1/8 tsp trace.
60 to 80 gallons: 3/4 tsp KNO3, 3/16 tsp KH2PO4, 1/4 tsp K2SO4, 1/4 tsp trace. 100 to 125 gallons: 1 1/2 tsp KNO3, 1/2 tsp KH2PO4, 1/2 tsp K2SO4, 1/2 tsp trace. The schedule attached to the chart is macros Monday, Wednesday, Friday, micros Tuesday, Thursday, Saturday, and a 50 percent or greater water change on Sunday.
Teaspoons are coarse. A level 1/8 teaspoon of KNO3 can vary by 20 percent between spoons, which is fine for EI because the method tolerates excess. For smaller tanks or for people who prefer liquids, dissolve a week's worth of each salt group in a bottle and dose by the milliliter. A dosing calculator such as Benchlet's converts a tank volume and target ppm into grams or milliliters, and remembers the tank so the numbers do not have to be retyped every water change day.
Why the 50 percent water change is not optional
The water change is the control on accumulation. If a tank receives dose D every week and half the water is replaced weekly, the concentration cannot climb without limit. The steady state ceiling is 2D: after many weeks the tank holds at most twice one weekly dose, because each change removes half of what has built up. With 20 ppm of nitrate dosed per week, the worst case is about 40 ppm, even if the plants ate none of it.
Skip the water change and the ceiling disappears. Skipping two weeks at full EI dosing in a tank with poor plant growth can push nitrate well past what many fish keepers consider comfortable. If large water changes are not practical, either dose less (a lean EI at 50 percent of the chart) or switch to a low daily method that keeps water column levels lower.
The water change also exports organic waste from fast growth and resets CO2 and pH, which is why EI tanks tend to look best on the day after the change. Match the fresh water temperature to the tank, dechlorinate, and dose the first macro dose right after refilling.
Reading the tank and adjusting
EI does not require test kits, but it does require observation. Green spot algae on old leaves and glass usually points to low phosphate or low CO2. Pale new growth with green veins suggests iron or trace shortage. Pinholes and yellowing old leaves point to potassium or nitrate. In every case the first thing to check is CO2, because nutrient symptoms at full EI dosing are almost always CO2 problems in disguise.
If algae appears with no nutrient deficiency symptoms, the usual cause is too much light for the CO2 supply, not too much fertilizer. Reduce light intensity or duration before cutting the schedule. Then, if the tank is stable and clean for a month, try reducing each dose by a quarter and watch for a response; the smallest dose that keeps plants free of deficiency signs is the right dose for that tank.
Frequently asked questions
What are the EI target levels?
Per week for a CO2 injected tank: about 20 ppm nitrate, 3 ppm phosphate, 20 to 30 ppm potassium, and 0.5 ppm iron, delivered across three macro doses and two or three micro doses.
Why are macros and micros dosed on different days?
Phosphate and iron can react to form insoluble iron phosphate, reducing the amount of each that stays available. Alternating days avoids dosing both at the same moment.
Can I do EI without a 50 percent water change?
Not at full strength. The weekly 50 percent change is what caps nutrients at about twice the weekly dose. Without it, dose much less or use a daily lean method.
Does EI work in a low tech tank without CO2?
Yes, at reduced amounts. Plants without CO2 grow slowly, so a quarter of the chart amounts, or one dose of each per week, is usually enough.
Do I need test kits for EI?
No. The method replaces testing with a known dose and a known water change. Test kits are useful only for troubleshooting an unexpected problem.
