Aquarium Fertilizer Dosing Calculator
Precise dry fertilizer doses for planted tanks — exact grams of KNO₃, KH₂PO₄, K₂SO₄, Epsom salt and CSM+B for EI, EI Lean, PPS-Pro or your own weekly targets.
Fertilizer dosing calculators
Tank volume
Use net water volume (subtract substrate and hardscape, roughly 15%). Don't know your net volume? Calculate it →
≈ 151.4 L / 40.0 gal
Choose a dosing regimen
| Fertilizer | Per dose | Per week |
|---|---|---|
Potassium Nitrate (KNO₃) Supplies NO3, K | 2.47g | 7.40g |
Monopotassium Phosphate (KH₂PO₄) Supplies PO4, K | 0.22g | 0.65g |
Potassium Sulfate (K₂SO₄) Supplies K | 1.11g | 3.33g |
Epsom Salt (MgSO₄·7H₂O) Supplies Mg | 5.12g | 15.36g |
Plantex CSM+B (trace mix) Supplies Fe | 0.36g | 1.08g |
Doses are dry-weight grams added directly to the tank. To dose from a bottle instead, switch to the Bottle method tab — it sizes a stock solution around the same weekly totals.
These targets are starting points, not prescriptions. Observe plant growth and algae over a few weeks and adjust; hobby test-kit accuracy varies widely, so trust the plants over the colour card.
Dosing rules
- • Never mix macros (P) and micros (Fe) in the same solution — they precipitate.
- • 50% water change every 7 days is mandatory for EI.
- • Shrimp keepers: verify CSM+B copper content is below 0.3%.
- • Dose macros in the morning when lights come on.
- • Store dry ferts sealed — KH₂PO₄ absorbs humidity fast.
Fertilizer coefficients
Ionic mass fraction of each compound. ppm added = grams × fraction × 1000 ÷ litres.
- KNO₃ → NO₃ 0.614 · K 0.387
- KH₂PO₄ → PO₄ 0.698 · K 0.287
- K₂SO₄ → K 0.449
- MgSO₄·7H₂O → Mg 0.099
- CSM+B → Fe 0.070
Example: 1 g of KNO₃ in 100 L raises NO₃ by 6.1 ppm.
EI vs PPS-Pro vs EI Lean — which dosing method?
All three regimens in this aquarium fertilizer dosing calculator use the same dry fertilizers — KNO₃, KH₂PO₄, K₂SO₄, Epsom salt and CSM+B. What changes is how much you add, how often, and how you deal with what the plants don't use.
EI (Estimative Index)
EI doses nutrients deliberately in excess so growth is never nutrient-limited, and caps accumulation with a 50% water change every week. That water change is not optional — it is the reset that makes the method safe to run blind, without testing. EI fits high-light tanks with pressurised CO2, where uptake is fast and the cost of running short on nitrate or phosphate is algae. It is the most forgiving method if you are inconsistent with testing, and the most demanding if you dislike weekly water changes.
EI Lean (low-tech)
EI Lean keeps the EI structure but cuts the targets to roughly a third. In a low-light tank without CO2 injection, plants simply cannot use full EI levels, and the surplus feeds algae instead. Lean dosing two or three times a week suits slow-growing setups — crypts, anubias, ferns, stem plants under modest light — and normal fortnightly maintenance water changes are enough.
PPS-Pro
PPS-Pro (Perpetual Preservation System) aims to match daily plant uptake rather than exceed it, so nutrient levels stay low and steady and no large weekly reset is required. It rewards consistency: small doses every day, ideally with occasional testing to confirm you are not drifting toward zero. It suits stable, moderately lit tanks and keepers who prefer smaller water changes over the brute-force approach.
Which one should you pick?
There is no winner here — plenty of award-winning tanks run each method. High light plus CO2 plus a willingness to do a weekly 50% change points to EI. Low tech, no CO2, and slow plants point to EI Lean. Daily routine, moderate light and a preference for lower standing nutrients point to PPS-Pro. Whichever you pick, give it four to six weeks before judging it, and change one variable at a time.
Dosing calculator FAQ
What's the difference between EI and PPS-Pro dosing?
EI (Estimative Index) deliberately doses nutrients in excess so plants are never limited, and resets accumulation with a 50% weekly water change. PPS-Pro doses small daily amounts intended to match plant uptake, so levels stay low and steady and large water changes are not required. EI suits high-light CO2 tanks; PPS-Pro suits moderate, stable setups where you prefer smaller water changes.
How much KNO3, KH2PO4 and K2SO4 do I add for my tank size?
Grams scale linearly with net water volume. As a reference, standard EI in a 190 L (50 gallon) tank needs about 9.3 g KNO3, 4.3 g KH2PO4 and roughly 3 g K2SO4 per week, split over three doses. Enter your own volume in the calculator above and it returns per-dose and per-week grams for KNO3, KH2PO4, K2SO4, Epsom salt and CSM+B.
How do I make a dry-fert dosing solution (bottle method)?
Weigh the weekly total of a compound, dissolve it in a measured volume of RO or distilled water (500 ml is common), shake until fully clear, and dose a fixed number of millilitres per dose. The bottle tab of this calculator gives grams per bottle and millilitres per dose, and warns you if the amount exceeds what will dissolve cleanly in the bottle size you picked.
Do I need to do water changes when dosing?
With EI, yes — a 50% weekly water change is part of the method because it caps nutrient accumulation. With EI Lean and PPS-Pro the dosed amounts are close to plant uptake, so water changes become normal maintenance rather than a requirement of the dosing schedule.
Why shouldn’t I mix macros and micros in the same bottle?
Phosphate from KH2PO4 reacts with the iron and other metals in a trace mix like CSM+B and forms insoluble precipitates, so part of the iron drops out of solution before it ever reaches your plants. Keep macros in one bottle and micros in another, and dose them on alternating days.
Is dosing safe for shrimp?
Macronutrients at EI levels are safe for shrimp. The risk is copper: CSM+B and most trace mixes contain copper, which is toxic to invertebrates at elevated levels. Dose traces at the lower end, never overdose the trace bottle, and check that your mix is around 0.1 to 0.3 percent copper before using it in a dedicated shrimp tank.
Not sure of your net volume?
Every gram on this page scales with water volume, so substrate and hardscape displacement matter.
Aquarium volume calculatorPlanning livestock too?
Check bioload and compatibility before adding fish to your planted scape.
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