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How to Dose Dry Fertilisers in a Planted Aquarium (EI Method)

Dry fertilisers are an economical way to feed a planted tank, and the Estimative Index (EI) method keeps dosing simple. The idea is to give your plants non-limiting nutrients, so they never run short. Precision isn’t critical. Being consistent is. EI deliberately doses a little more than your plants need. A 50% water change resets the tank, and it’s part of the method.

What you’ll need

Why two containers? Your macro salts (KNO₃, KH₂PO₄, K₂SO₄) are needed in grams per dose, while your micro/trace mix (Rexolin APN) is needed in fractions of a gram. If you put everything in one bottle, the tiny amount of trace elements gets swamped by the large volume of macro solution, and it becomes very hard to dose the micros accurately on their own schedule. Keeping them separate means you can adjust each independently as your tank’s needs change.

What do you do with them? You dissolve the dry salts into water to make a liquid solution in each bottle, then dose a measured amount (a set number of pumps, or a syringe reading) into the tank on your chosen days. Once the solution is mixed, you never weigh powders again — you just squirt a set amount from the bottle.

Step 1: Work out your tank’s water volume

The calculators work in litres of water, not the size on the tank’s label. Substrate and hardscape take up space, so your real water volume is lower than the label says.

Step 2: Choose a calculator

Both calculators do the same job. You give them your tank volume and the dosing method you want to follow, and they show how much of each fertiliser to use. Use whichever you find easier.

Option A: the IFC Aquarium Fertilizer Calculator (spreadsheet)

This Excel spreadsheet was made by members of the UKAPS forum. It isn’t ours, but it’s the one we use and recommend. You can find it on the UKAPS forum. You only need a few of its tabs:

  1. TankAndDosing: set your tank size, the size of your dosing container (how much solution you make up at once), how much of it you add each time, and how many times a week.
  2. Target Calculator: this is where you choose your dosing method. There’s a dropdown where you can select “EI” (or other methods like PPS Pro) to clone the standard recipe — the calculator fills in all the target levels for you automatically. You don’t need to type in individual numbers. Once your method is selected, read the “Add to Container (gram)” column. It shows how much of each fertiliser to add to each container. Container 1 is for macros (potassium nitrate, monopotassium phosphate and potassium sulphate) and Container 2 is for micros (a trace mix such as Rexolin APN). For magnesium sulphate and calcium sulphate, follow the calculator: it tells you whether each one goes in a container or straight into the tank. To dose dry instead, use the “Dry Dosing” column.
  3. Core Settings (optional): enter your local tap water report so the amounts allow for what’s already in your water. You don’t need this to get started.

Watch for red writing in the spreadsheet. It’s a warning, for example that a compound shouldn’t go into a container.

Option B: the Rotala Butterfly nutrient calculator

This is an online calculator, and the one we point to on our product pages. Open it at rotalabutterfly.com/nutrient-calculator.php.

In the “I am calculating for” dropdown, select EI (or another method like PPS Pro or EI Low Light). The calculator then shows you the target level for each nutrient and how much of each fertiliser to use. You work through the salts one at a time.

Below the results, the calculator shows a visual bar graph that places your dose on a scale alongside the ranges for EI, PPS Pro, and other methods. It’s a quick visual check that you’re in the right ballpark without needing to interpret numbers.

It’s another site we don’t control, so follow its own on-screen instructions.

Watch out for potassium. Potassium nitrate and monopotassium phosphate both add potassium as well, so count that first, then work out how much potassium sulphate you still need to reach your total potassium target. The IFC calculator handles this automatically. With Rotala Butterfly, you need to do it yourself — calculate KNO₃ and KH₂PO₄ first, note the potassium they add, then subtract that from your total potassium target before calculating K₂SO₄.

Working out your amounts

Your calculator does the maths, so we haven’t listed amounts here. They depend on your tank and the method you’re following. For a worked example, see the EI Dosing Pack page.

Step 3: Mix your solutions

  1. Take your macro bottle (500 mL or whatever size your calculator specifies). Add the dry macro salts one at a time, swirling or shaking until each one is fully dissolved before adding the next.
  2. Take your micro bottle (smaller is fine, 100–250 mL). Add your trace mix (Rexolin APN) and dissolve.
  3. If your calculator says magnesium sulphate and/or calcium sulphate go straight into the tank rather than a container, dissolve them in a small cup of tank water and pour in directly on dosing day.
  4. Label your bottles with the date you mixed them and the dosing amount.

Your solutions are now ready. On dosing days, you simply pump out the measured amount from each bottle and add it to the tank. No weighing, no measuring dry powder, no mess.

A typical week

  1. Do your 50% water change. It resets the tank.
  2. Dose your macro and micro containers on your chosen days, in the amounts your calculator gives you.
  3. Add magnesium sulphate and calcium sulphate the way your calculator says, either from a container or straight into the tank.

Before you start

  • Check the form of each salt. Magnesium sulphate usually comes as the heptahydrate (MgSO₄·7H₂O) and calcium sulphate as the dihydrate (gypsum, CaSO₄·2H₂O), and the amounts differ between forms. Choose the form in your calculator that matches your product.
  • Start with the amounts your calculator gives you and watch your plants. EI is designed to be forgiving.
  • Keep fertilisers sealed in a cool, dry place, away from children and pets, and wear gloves when you mix.

Need a hand?

Message us through our contact page and we’ll be happy to help you set up your dosing.

The IFC calculator is the work of UKAPS forum members and isn’t produced by Green Aquarium. The Rotala Butterfly calculator is an independent website.

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