Planted-aquarium fertilizer calculator
Convert salts, solutions, and doses for planted-tank fertilization. Pick a salt, enter your tank volume and stock solution size, and see exactly what each millilitre adds to the water.
What 1 ml of solution does in your tank
At 1 ml into 100 L:
- NO3 rises by +0.5029 mg/L
- K rises by +0.3171 mg/L
Dosing daily:
- NO3: +3.5203 mg/L weekly
- K: +2.2197 mg/L weekly
Enter how much salt you mix into a solution of given volume. We show how much of each element reaches the tank per milliliter of dose.
Save this solution in AquaPilot
Keep the recipe next to your tank — attach it to doses and track how parameters respond over time.
How it works
- 1
Pick a salt
Choose the fertilizer salt you have on hand. The calculator knows the molar mass and the elements each salt releases.
- 2
Set tank and solution volume
Enter your aquarium volume in litres and how much stock solution you want to prepare in millilitres.
- 3
Read the dose
You'll see how many mg/L of each element a single millilitre of the solution adds to your tank — plus a daily/weekly projection.
When to use it
Mixing DIY macro or micro solutions
Work out how much KNO₃, KH₂PO₄, or a chelated micro mix to dissolve to hit the concentrations you target per dose.
Switching from a commercial fertilizer
Back-calculate what a commercial dose actually adds and match it with your own salts at a fraction of the cost.
Troubleshooting parameters
See how a given dose affects NO₃, PO₄, K, Mg or Ca — useful when plants show deficiencies or a parameter is drifting.
A quick primer on aquarium fertilization
Planted aquariums need a steady supply of macronutrients (NO₃, PO₄, K) and micronutrients (Fe, Mn, Zn, Cu, B, Mo). The right dose depends on tank volume, plant mass, light intensity and CO₂ — the calculator above handles the arithmetic so you can focus on the strategy.
Macronutrients
Elements plants consume in measurable quantities — typically dosed in mg/L (ppm) weekly. Common target ranges for moderately lit planted tanks:
- NO₃ (azot)10–20 mg/L
- PO₄ (fosfor)0.5–2 mg/L
- K (potas)10–20 mg/L
- Mg (magnez)5–10 mg/L
- Ca (wapń)10–30 mg/L
Micronutrients
Trace elements needed in tiny amounts — usually dosed via a chelated mix (DTPA or EDTA). Excess can stain water or poison invertebrates.
- Fe (żelazo)0.1–0.5 mg/L (weekly)
- Mn, Zn, Cu, B, Motrace — follow mix label
Common dosing methods
Two battle-tested approaches for hobbyists dosing their own fertilizers. Both work — the difference is how much you dose and how often you change water.
EI (Estimative Index)
Tom Barr's method — dose macros in excess (e.g. 20 ppm NO₃ weekly) and reset with a 50% weekly water change. Fast, forgiving, great for high-tech tanks.
PPS-Pro
Perpetual Preservation System — dose small amounts daily to match plant uptake. Requires parameter testing and fewer water changes. Good for low-tech and low-maintenance setups.
Common fertilizer salts
Typical salts used in DIY aquarium fertilizers, with molar mass (used by the calculator) and the elements they release into the water.
| Formula | Molar mass | Releases |
|---|---|---|
| KNO₃ | 101.10 g/mol | K, NO₃ |
| KH₂PO₄ | 136.09 g/mol | K, PO₄ |
| K₂SO₄ | 174.26 g/mol | K |
| MgSO₄·7H₂O | 246.47 g/mol | Mg, SO₄ |
| CaCl₂·2H₂O | 147.01 g/mol | Ca, Cl |
| Ca(NO₃)₂·4H₂O | 236.15 g/mol | Ca, NO₃ |
Hydrated forms (e.g. MgSO₄·7H₂O, CaCl₂·2H₂O) include water of crystallization, which increases the molar mass. Always check the label on what you buy.
Worked examples
Three common dosing questions and the exact amount of salt you need — add directly to the tank or dissolve first into a stock solution.
| Question | Salt | Amount |
|---|---|---|
| Raise NO₃ by 5 mg/L in a 100 L tank | KNO₃ | ~0.82 g |
| Raise PO₄ by 0.5 mg/L in a 200 L tank | KH₂PO₄ | ~0.14 g |
| Raise K by 10 mg/L in a 150 L tank | K₂SO₄ | ~3.34 g |
Use the calculator above to work out the equivalent dose as a diluted stock solution (e.g. 5 g in 500 ml, dosed at 1 ml per litre).
Frequently asked questions
What unit is the result in?
Milligrams per litre (mg/L), which for dilute aquarium solutions is equivalent to ppm. Each number shows the increase per dose, not the absolute tank concentration.
Does the calculator account for existing levels in my tank?
No — it shows how much a dose raises each element. To estimate the final level, add the rise to your most recent test reading.
Why does my salt raise more than one element?
Many fertilizer salts release multiple elements (e.g. KNO₃ adds both K and NO₃). The calculator lists every element the salt contributes so you can see side effects.
Can I use it for saltwater or brackish tanks?
The math works for any volume, but the suggested salts and typical dose ranges are tuned for freshwater planted tanks. Use with care outside that context.
Is my data saved?
Not on this page — it's a standalone calculator. Create an AquaPilot account to save recipes, link them to dosing events, and track parameter trends over time.
Related tools
Want AquaPilot to do this automatically?
Log measurements, dosing and maintenance history, and the app will help you tune doses and keep parameters stable.
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