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Aquarium Fertilizer Dosing Calculator

Nutrient deficiency and algae outbreaks share a common cause: inconsistent fertilization — this calculator gives you a weekly dosing schedule matched to your tank size and plant load.

A planted aquarium burns through three macronutrients — nitrogen, phosphorus and potassium — plus a mix of trace metals led by iron. How fast it burns through them depends on three things: how much water you have, how heavily the tank is planted, and whether you inject CO2. Get the supply matched to the demand and growth is steady, color is strong and algae has nothing to feed on.

The schedule below is built from dry salts — potassium nitrate (KNO3), mono-potassium phosphate (KH2PO4), potassium sulphate (K2SO4) and a comprehensive trace mix such as CSM+B. Dry salts are the cheapest and most precise way to dose, and every popular method uses the same four ingredients in different ratios and frequencies.

Pick a method, enter your tank, and you get exact gram weights per dose, the days to dose them, and the weekly water change that keeps everything in range. Treat the numbers as a confident starting point, then read your plants over the next month and fine-tune.

Tank Volume

40.0 gal · 151 L

Plant Density

Dosing Method

CO2 Injection

Frequently Asked Questions

What is the Estimative Index (EI) dosing method?
Estimative Index, developed by Tom Barr, deliberately keeps nutrients in excess so plants are never limited. You dose macros (nitrogen, phosphorus, potassium) and micros on alternating days, then reset everything with a 50% water change once a week. Because nutrients stay above the level plants can consume, you never have to test the water — the weekly water change prevents buildup. EI suits high-light, CO2-injected tanks with fast-growing plants.
How is lean dosing different from EI?
Lean dosing keeps nutrients near the minimum plants need rather than in excess. Nitrate is held low (around 5-10 ppm), phosphate very low (0.1-0.5 ppm), and water changes can be smaller and less frequent. The philosophy is that lower nutrients — especially phosphate — combined with strong CO2 and good flow produce healthier coloration in red plants and less algae pressure. Lean dosing demands consistency and usually some water testing.
Do I need more fertilizer if I inject CO2?
Yes. CO2 dramatically increases the rate of photosynthesis, so plants pull nutrients out of the water far faster. A CO2-injected high-light tank can consume several times the nitrogen, phosphorus, and potassium of an identical low-tech tank. This calculator raises the recommended doses when you enable CO2 so growth is not nutrient-limited.
What are the target nutrient levels for a planted tank?
For most planted tanks: nitrate (NO3) 10-25 ppm, phosphate (PO4) 1-2 ppm, and potassium (K) 10-30 ppm. Iron and trace elements come from a comprehensive micro mix (like CSM+B). Lean-dosing tanks intentionally sit at the bottom of these ranges or below. These are guidelines, not hard limits — observe your plants and adjust.
Why dose macros and micros on separate days?
Phosphate and many trace metals (especially iron) can react and precipitate out of solution when mixed at high concentration, making the iron unavailable to plants. Dosing macros (KNO3, KH2PO4, K2SO4) on one set of days and micros (CSM+B) on alternating days avoids that reaction in the water column. With separate dry-dosing this is the standard EI schedule.
Can I use this for a nano tank under 10 gallons?
You can, but dry-dosing tiny amounts accurately is hard — a 5-gallon dose may be a fraction of a gram. For nano tanks, mix a stock solution (dissolve a measured amount of dry salts in water) and dose by milliliters, or use a commercial all-in-one liquid. Always start at half the calculated dose and watch for algae before increasing.

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