Planted Tank Fertilizer Dosing Calculator (EI)
Enter your tank size to get dry Estimative Index dosing amounts and a full weekly schedule: macros on Monday, Wednesday, Friday, micros on Tuesday, Thursday, Saturday, and a 50 percent water change every Sunday.
Use your real water volume, which is roughly 90% of the tank label once substrate and hardscape are in. Not sure? Run the volume calculator first.
For a -gallon tank, your EI bracket is .
Per-dose amounts
0
KNO3 (macro)
0
KH2PO4 (macro)
0
CSM+B (micro)
Your weekly schedule
| Day | Dose | Amount |
|---|
Low light or no CO2? EI deliberately overdoses and resets with the weekly 50% water change, so it really only pays off with strong light and CO2. Low-tech tanks should dose lean, about half these amounts or less, or you will just feed algae. Dose macros and micros on alternate days, never the exact same moment.
Fertilizers for this schedule
Dry salts are the economical, precise choice for EI; an all-in-one liquid is the simple route.
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What Is Estimative Index Dosing?
Estimative Index, almost always shortened to EI, is a fertilizing method for planted aquariums built on a simple idea: rather than testing your water constantly and trying to hit exact nutrient targets, you add a generous, known amount of nutrients on a fixed schedule so plants are never short of anything, then perform a large weekly water change to wipe the slate clean. The method splits nutrients into two groups. Macronutrients are nitrogen and phosphorus, dosed as potassium nitrate (KNO3) and monopotassium phosphate (KH2PO4). Micronutrients are the trace elements like iron and manganese, dosed as a CSM+B mix. This calculator picks the right bracket for your tank size and lays out exactly how much of each to add and on which day.
The Weekly Rhythm: Macros, Micros, and a Reset
EI runs on a steady weekly rhythm. Macronutrients go in on Monday, Wednesday, and Friday. Micronutrients go in on Tuesday, Thursday, and Saturday. The reason for alternating them is practical: phosphate from KH2PO4 and some of the trace metals in CSM+B can react with each other if dosed together, so keeping them a day apart avoids that. Then on Sunday you change 50 percent of the water. That single large water change is the backbone of the whole method, because it removes any nutrients that built up over the week and returns the tank to a low, consistent baseline before the next round of dosing starts. Skip the water change and nutrients simply climb week after week, which is the opposite of what you want.
Why EI Overdoses on Purpose
The deliberate excess is the point. By keeping nutrients in surplus, EI removes the guesswork and the risk of a plant stalling because it ran out of one element. That works beautifully when plants are growing fast, which in turn requires strong light and injected CO2. In a high-tech tank, fast growth consumes the surplus and the weekly water change handles the rest. The catch is that this only makes sense when plants can actually use the nutrients. Pour EI levels into a slow-growing, low-light tank and the plants take up only a fraction of it, leaving the rest to fuel algae. That is the single most common mistake new keepers make with EI.
Adjusting for Low-Light and No-CO2 Tanks
If your tank is low-tech, with modest lighting and no pressurized CO2, you should dose lean. Cut the chart amounts to roughly half, or even less, keep the same macro and micro day pattern, and watch your plants over a few weeks. Steady, healthy new growth tells you the dose is about right, while creeping algae or stalled plants tell you to pull back. You can still run the weekly water change, which keeps the tank clean and stable regardless of how heavily you dose. The goal is to match nutrient supply to how fast your plants are actually growing, and in a low-tech tank that pace is gentle. Pair your dosing with the right lighting and, if you inject gas, the CO2 calculator, and keep water changes on schedule so the whole system stays balanced.
Keep going: round out your planted-tank setup.
Frequently Asked Questions
What is Estimative Index (EI) dosing?
Estimative Index is a planted-tank fertilizing method that deliberately adds more nutrients than the plants can use, so they are never starved, then resets the water column with a large weekly water change. Instead of testing and chasing precise numbers, you dose macronutrients (nitrogen and phosphorus as KNO3 and KH2PO4) on some days and micronutrients (a CSM+B trace mix) on others, then change 50 percent of the water once a week to clear any buildup.
Do I need CO2 to run EI dosing?
EI was designed for high-light, CO2-injected tanks where plants grow fast and demand a steady flood of nutrients. In that setting the heavy dosing makes sense. If you run full EI on a low-light tank with no CO2, the plants cannot use all those nutrients and you simply feed algae instead. For low-tech tanks, dose lean: roughly half the chart amounts or less, and watch how your plants respond before adding more.
Should I use dry or liquid fertilizers?
Dry salts like KNO3, KH2PO4, and CSM+B are far cheaper per dose and let you dial in exactly how much nitrogen, phosphorus, and trace you add, which is why EI keepers favor them. A small bag of each can last a year or more. Premixed liquid all-in-one fertilizers cost more but are convenient and beginner-friendly, with no measuring of tiny teaspoon fractions. Both work. Dry is economical and precise, liquid is simple.
How do I dose lean for a low-light tank?
Lean dosing means cutting the standard EI amounts roughly in half, or even lower, because a low-light tank without CO2 grows slowly and needs far fewer nutrients. Start at about half the chart dose for your tank size, keep the same macro and micro day schedule, and observe your plants for a few weeks. Healthy new growth means you are close. Algae or stalled plants tell you to adjust. You can still do the weekly water change.
Is EI dosing safe for shrimp and snails?
Standard EI is generally considered safe for snails and most shrimp at the dosing levels on the chart, since the nutrients are plant fertilizers rather than toxins. The bigger concern for sensitive shrimp is copper, which is present in trace mixes like CSM+B, though the amounts in normal dosing are very low. Sensitive species such as crystal or other Caridina shrimp are more cautious keepers dose lean and rely on stable parameters and consistent water changes.
Why does EI use a 50 percent weekly water change?
Because EI deliberately overdoses, nutrients would slowly accumulate week after week without a reset. The large 50 percent weekly water change clears that excess and returns the tank to a known, low baseline before the next week of dosing begins. Skipping it lets nutrients climb and defeats the whole purpose of the method. Pick one day, traditionally Sunday, change half the water, then resume the macro and micro schedule the following day.
When during the day should I add fertilizer?
Most keepers dose in the morning, around when the lights and CO2 come on, so nutrients are available throughout the photoperiod when plants are actively growing. Add macros on their days and micros on theirs, ideally not at the exact same moment, because phosphate and some trace metals can react together. Spacing macro and micro doses to alternate days, as this schedule does, neatly avoids that interaction.