Aquarium Fertilizer Calculator (EI Dry Ferts)
Work out how many grams of each dry fertilizer salt your planted tank needs. Build a full Estimative Index (EI) schedule, find the dose for a specific ppm, or check the strength of a homemade stock solution.
How to use this calculator
- Enter your real water volume, not the tank's nominal size. The aquarium volume calculator can estimate it.
- Pick a mode. "EI weekly plan" gives a complete schedule. "Dose for a target ppm" works out a single salt. "Stock solution" tells you how many ppm each mL of a homemade liquid adds.
- Check the iron percentage of your trace mix. The current Plantex CSM+B label lists 7% chelated iron. Older batches were sold as 6.53%, so read your own bag.
- Weigh with a 0.01 g scale. A cheap jewelry scale is accurate enough and far more consistent than spoons.
How Estimative Index dosing works
The Estimative Index, developed by Tom Barr, aims to keep every nutrient at a level that is never limiting for plants in a high-light, CO2-injected tank. Instead of testing, you add a little more than plants can use, then do a large weekly water change so nothing builds up out of control. With a 50% weekly water change, nutrient levels can never climb above about twice what you add each week.
A typical week looks like this:
| Day | Dose |
|---|---|
| Sunday | 50% water change, then macros |
| Monday | Micros (CSM+B) |
| Tuesday | Macros |
| Wednesday | Micros |
| Thursday | Macros |
| Friday | Micros |
| Saturday | Rest day |
Keep macros and micros on separate days. Phosphate can bind with the iron and other metals in the trace mix if they are added together.
Formulas used
Every salt contains a fixed fraction of the nutrient you want, based on its molar mass. To raise a nutrient by a set ppm (which equals mg per liter):
And to find the ppm a dose adds:
In the EI plan, KNO3 and KH2PO4 already add potassium. The calculator counts that first and only uses K2SO4 to make up the rest of the potassium target.
Nutrient content of common dry ferts
| Salt | Molar mass | Nutrient content |
|---|---|---|
| KNO3 (potassium nitrate) | 101.10 g/mol | 61.3% NO3, 38.7% K |
| KH2PO4 (monopotassium phosphate) | 136.08 g/mol | 69.8% PO4, 28.7% K |
| K2SO4 (potassium sulfate) | 174.25 g/mol | 44.9% K |
| MgSO4·7H2O (Epsom salt) | 246.47 g/mol | 9.86% Mg |
| Plantex CSM+B | mixture | 7% Fe, 2% Mn, 1.5% Mg, 0.4% Zn, 0.1% Cu, 0.06% Mo (label) |
Worked example: 20 gallon high-tech tank
With 20 US gallons (75.7 liters) of water and weekly targets of 20 ppm NO3, 3 ppm PO4, 30 ppm K, 10 ppm Mg and 0.5 ppm Fe:
- KNO3: 20 × 75.7 ÷ 1,000 ÷ 0.613 = 2.47 g per week, or 0.82 g per dose.
- KH2PO4: 3 × 75.7 ÷ 1,000 ÷ 0.698 = 0.33 g per week, or 0.11 g per dose.
- K2SO4: the KNO3 and KH2PO4 already add 13.8 ppm K, so only 16.2 ppm more is needed: 2.73 g per week, or 0.91 g per dose.
- MgSO4·7H2O: 7.68 g per week, or 2.56 g per dose (skip it if your tap water has enough magnesium).
- CSM+B: 0.5 × 75.7 ÷ 1,000 ÷ 0.07 = 0.54 g per week, or 0.18 g per dose.
Stock solutions
Many people dissolve a week's or a month's worth of salts in distilled or RO water and dose with a syringe. For example, 50 g of KNO3 in a 500 mL bottle holds 0.1 g per mL. A 5 mL dose (0.5 g) in 20 gallons adds 4.05 ppm NO3 and 2.55 ppm K. If crystals stay at the bottom, the solution is saturated: add more water and redo the calculation with the new bottle volume. Store trace mixes in the fridge to slow mold growth.
Why grams and not teaspoons?
Teaspoon charts are popular, but they are not consistent. Members of the UK Aquatic Plant Society found that "teaspoon" volumes range from about 3.5 to 5.9 mL depending on the spoon, and that the weight of a spoonful changes with grain size, humidity and whether the spoon is level. Different calculators also use different densities for the same salt. Weighing removes all of that guesswork.
Frequently asked questions
What are the EI fertilizer targets?
A commonly quoted set of weekly Estimative Index targets is 20 ppm nitrate, 3 ppm phosphate, 30 ppm potassium, 10 ppm magnesium and 0.5 ppm iron. Other guides use ranges of 20–30 ppm nitrate and 1–3 ppm phosphate. EI is meant to give non-limiting levels, so exact numbers matter less than consistency and the weekly water change.
Why does this calculator use grams instead of teaspoons?
A teaspoon of fertilizer salt can weigh very different amounts depending on the spoon size, how finely the salt is ground, whether the spoon is level and how much moisture the salt has absorbed. Planted tank forums report teaspoon volumes ranging from about 3.5 to 5.9 mL. Weighing with a 0.01 g scale is far more consistent.
Do I need to dose magnesium?
Only if your water is low in it. Many tap waters already supply enough magnesium. If you use RO water or have very soft tap water, dose Epsom salt or a GH booster. Otherwise set the magnesium target to 0.
Should I use the tank size or the real water volume?
Use the real water volume. Substrate, rock and the gap below the rim mean most tanks hold 10 to 20 percent less water than their nominal size. Dosing for the nominal size slightly overdoses, which EI tolerates, but the real volume gives more accurate ppm figures.
Is EI safe without CO2?
EI was designed for high-light tanks with injected CO2, where plants use nutrients quickly. In low-light tanks without CO2, plants grow slowly, so full EI doses mostly build up in the water. For low-tech tanks, use a much smaller dose and lower targets, and watch how the plants respond.
Related calculators
- CO2 calculator: check your CO2 level from pH and KH
- Water change calculator: see how water changes cap nutrient build-up
- Aquarium volume calculator: find your real water volume
Sources: Molar masses from PubChem: potassium nitrate, monopotassium phosphate, potassium sulfate, magnesium sulfate heptahydrate. Trace mix analysis from Plant Products, Plantex NutriTrace CSM and Barr Report discussion of CSM+B composition. EI weekly targets as quoted from the UKAPS EI article in UKAPS, "Recommended ppm". EI water change principle from Barr Report, "The Estimative Index – What is it?". Teaspoon variability from UKAPS, "Teaspoons to grams conversion".
Last updated: October 2026