Fertiliser Dosing Calculator
Fertiliser Dosing Calculator generates a complete weekly EI schedule or liquid dose for your tank volume — switch between modes using the toggle above.
Calculator
Low
Sparse planting, mostly fish — 75% standard dose
Medium
Mixed planting, moderate growth — standard dose
High
Dense, fast-growing plants under CO2 — 125% dose
High density means most substrate covered with fast-growing plants under CO2 injection
The 50% weekly water change is a core part of EI — it resets accumulated nutrients
Enter your values above to see results
Fertilisers for planted tanks
Liquid fertilisers for all skill levels and an all-in-one EI starter kit.
All-in-one liquid fertiliser with nitrogen — suitable for low to medium light planted tanks
Higher-nutrient liquid formula for CO2-injected tanks — use when plants show signs of deficiency
Pre-measured dry salt EI kit — takes the guesswork out of Estimative Index dosing
I’ve used everything listed in my own tanks. If you buy through a link, I earn a small commission — at no extra cost to you, and it never affects what I recommend.
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Frequently Asked Questions
- What is Estimative Index (EI) dosing?
- Estimative Index dosing is a fertilisation method developed by Tom Barr that works by adding a known excess of nutrients throughout the week, then performing a large water change — typically 50% — every Sunday to reset accumulated nutrients to a baseline. Rather than trying to precisely match nutrient uptake, EI intentionally doses more than the plants can consume and relies on the water change to prevent toxicity. This approach is highly effective for CO2-injected planted tanks and removes the need for constant water testing to dial in doses.
- How often should I fertilise my planted aquarium?
- In the EI method, macronutrients (KNO3, KH2PO4, K2SO4) are dosed on Monday and Friday, and micronutrients (trace elements) on Wednesday and Saturday. This alternating schedule prevents interactions between macro and micro elements. For liquid all-in-one fertilisers, weekly dosing is typical — some hobbyists split this into smaller daily doses for stability, especially in high-tech tanks. The calculator gives you both weekly and daily amounts.
- What fertilisers do I need for a planted tank?
- Plants need macronutrients (nitrogen, phosphorus, potassium) and micronutrients (iron, manganese, zinc, and other trace elements). For EI dosing you typically use KNO3 for nitrogen and potassium, KH2PO4 for phosphorus, K2SO4 as an extra potassium supplement, and a commercial trace element mix for micros. Low-tech tanks can often get by with an all-in-one liquid fertiliser like Tropica Specialised, which contains all essential elements in one bottle.
- Why do I need a 50% water change with EI dosing?
- The 50% Sunday water change is fundamental to the EI method, not optional. Because EI doses nutrients in excess intentionally, without the weekly reset, nutrients would accumulate over time to potentially toxic levels. The large water change dilutes any build-up and resets the tank chemistry to a known baseline, allowing the following week's dosing to start fresh. Some hobbyists reduce this to 40% for large tanks or increase it to 60% for heavily planted high-light setups.
- Can I use EI dosing without CO2?
- EI dosing is designed for high-tech planted tanks with CO2 injection, where plant growth is fast and nutrient demand is high. Without CO2, plants grow much more slowly and cannot absorb nutrients at the same rate. Dosing at full EI levels without CO2 will cause algae problems rather than lush growth. If you have a low-tech tank without CO2, use the Liquid Fertiliser mode instead and halve the manufacturer's recommended dose — or use Tropica Nutrition (without specialised) which is lower in nutrients for low-tech setups.