Aquarium Fertilizer Calculator: Calculate the Right Fertilizer Dosage for Your Aquarium
Use this tool to calculate the correct fertilizer dosage for your aquarium plants. Fertilizing your plants properly ensures they grow healthy and vibrant, while avoiding over or under-fertilization.
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How to Calculate an Aquarium Fertilizer Dose
Fertilizer dosing comes in two forms: commercial liquid products (dosed per ml per gallon), and dry salts (dosed to a target ppm using molecular weight). The two do not share a formula. This guide gives both, the target ppm for macro- and micronutrients, and the practical framework (EI vs PPS) that determines when you dose.
Two Dosing Paths
Path 1 — Commercial liquid product dose_ml = (target_ppm × volume_L) / (product_ppm_per_ml) Path 2 — Dry salt (DIY) mass_mg = (target_ppm × volume_L) / element_fraction Then convert to grams: mass_g = mass_mg / 1000
Path 1 uses the product's labeled ppm-per-ml (many products list ppm contributed per 1 ml in 10 gal — convert to ppm-per-ml first). Path 2 uses the salt's element fraction: the fraction of the salt's mass that is the target nutrient (e.g., KNO₃ is 61.3% NO₃⁻ by mass).
Worked Example — Dry Salt
Target 10 ppm NO₃⁻ in a 40-gallon tank using KNO₃ (potassium nitrate):
Volume = 40 gal × 3.785 = 151.4 L KNO₃ fraction of NO₃⁻ ≈ 0.613 (61.3% by mass) mass_mg = (10 × 151.4) / 0.613 ≈ 2,470 mg mass_g = 2,470 / 1000 = 2.47 g → Dose ~2.47 g of KNO₃ to raise NO₃⁻ by 10 ppm in 40 gal.
Compare to Path 1 (commercial product): if a liquid product contributes 1.0 ppm NO₃⁻ per ml in 10 gal, then in 40 gal the same 1 ml raises NO₃⁻ by 0.25 ppm. To raise by 10 ppm in 40 gal you need 40 ml.
Target PPM — Macro and Micro
| Nutrient | Typical target | Notes |
|---|---|---|
| NO₃⁻ (nitrate) | 5–30 ppm | Higher for fast-growing stems and EI |
| PO₄³⁻ (phosphate) | 1–3 ppm | Keep N:P ratio in mind — see algae note |
| K⁺ (potassium) | 10–30 ppm | Often dosed separately from N and P |
| Mg²⁺ | 5–15 ppm | Often in GH booster or tap water |
| Ca²⁺ | 20–40 ppm | Often in tap water or GH booster |
| Fe (iron) | 0.1–0.5 ppm | Trace-mix target; not a macro |
| Mn, Zn, Cu, B, Mo | Trace (µg/L to low mg/L) | Use a commercial trace mix — do not attempt individual salts |
These are starting points, not fixed rules. EI uses the higher end of each range with 50% weekly water changes; PPS-Pro uses the lower end with smaller, more frequent doses. The right level depends on light, CO₂, plant mass, and growth rate — watch plant response and adjust.
EI vs PPS-Pro — The Two Common Methods
| Aspect | Estimative Index (EI) | PPS-Pro |
|---|---|---|
| Macro dosing | High (NO₃⁻ 20–30 ppm after dose) | Low, precise (NO₃⁻ 5–10 ppm) |
| Frequency | 3× per week or every other day | Daily small doses |
| Water change | 50% weekly (required) | Smaller, less frequent |
| Best for | High light + high CO₂ fast-growth tanks | Low–medium light, stability-focused setups |
| Main risk | Algae if CO₂/light not matched; needs discipline on water changes | Deficiencies if under-dosed; slower growth |
Both methods work when applied consistently. Mixed or ad-hoc dosing (part EI, part PPS) is where imbalances often arise. Pick a framework, dose consistently, and adjust from test results and plant response — not from a single number.
Common Dry Salts and Their Element Fractions
| Salt | Contributes | Element fraction |
|---|---|---|
| KNO₃ | NO₃⁻ + K⁺ | 61.3% NO₃⁻; 38.7% K⁺ |
| KH₂PO₄ | PO₄³⁻ + K⁺ | 69.8% PO₄³⁻; 28.7% K⁺ |
| K₂SO₄ | K⁺ + SO₄²⁻ | 44.9% K⁺; 55.1% SO₄²⁻ |
| MgSO₄·7H₂O | Mg²⁺ | 9.9% Mg²⁺ |
| CaSO₄·2H₂O | Ca²⁺ | 23.3% Ca²⁺ |
| CSM+B | Fe + other traces | ~6.5–7.0% Fe (varies by batch — check label) |
Fractions are approximate and depend on the specific compound and hydration state. Use the label on your salt for precise values. For trace mixes (CSM+B), the Fe fraction is the reference — do not attempt individual trace salts unless you have a lab-grade scale and chemistry background.
Common Commercial Products
| Product | Type | Notes |
|---|---|---|
| Seachem Flourish | Micronutrient mix | Trace only; dose macros separately |
| Seachem Flourish Trace | Trace-only (no Fe) | Used with Flourish Iron |
| Seachem Flourish Nitrogen | N source | Dose to target NO₃⁻ with test kit |
| Seachem Flourish Phosphorus | P source | Dose to target PO₄³⁻ |
| Seachem Flourish Potassium | K source | Dose to target K⁺ |
| Aquarium Co-Op Easy Green | All-in-one | Dosed per pump; includes macros + traces |
| NilocG Thrive / Thrive+ | All-in-one | EI-style dosing; check label for target ppm |
All-in-one products simplify dosing but give less control over individual nutrients. If you test and see a persistently low nutrient, a targeted product (or dry salt) is more precise. Read the label ppm-per-dose figure and convert to ppm-per-ml-per-gallon before using the formula above.
Common Nutrient Deficiencies — Diagnose Before You Dose
Deficiency symptoms look similar across nutrients. Before increasing any dose, use this rough guide and confirm with a test kit where possible:
| Symptom | Likely cause |
|---|---|
| Old leaves yellow first, slow growth | Nitrogen (N) deficiency |
| Dark green / purple leaves, slow growth | Phosphorus (P) deficiency |
| Holes in leaves, brown edges | Potassium (K) deficiency, or snail damage |
| New leaves yellow between green veins | Iron (Fe) deficiency |
| Old leaves yellow between veins | Magnesium (Mg) deficiency |
| Stunted new growth, distorted tips | Calcium (Ca) or boron (B) deficiency |
| Uniform pale color, no growth | Insufficient light or CO₂ (not a nutrient issue) |
Two cautions: (1) many deficiencies look alike — confirm with a nitrate, phosphate, iron, or GH test before dosing. (2) adding more of the wrong nutrient shifts the balance toward algae. Test first, dose second.
What About Algae?
A common belief is that "excess fertilizer causes algae." That is only partly true. In modern planted-tank practice, the dominant factors in algae are:
- Nutrient imbalance (e.g., very high N relative to P, or vice versa) — not just high levels
- Light intensity exceeding CO₂ and nutrient availability — the most common cause
- Unstable CO₂ — fluctuating levels are worse than steady high levels
- Low plant mass during early setup — plants do not yet compete with algae
The EI method deliberately doses "high" macros — yet many EI-run tanks have little algae, because CO₂, light, plant mass, and flow are also well-managed. The lesson is not "feed less" but "keep the four inputs (light, CO₂, nutrients, plant mass) in balance." See the CO2 calculator and light wattage calculator for the other two inputs.
Related Tools
- CO2 calculator — CO₂ injection rate
- Light wattage calculator — light input for the balance
- Dosage calculator — general dosing for medications and treatments
- Volume calculator — actual water volume
- Substrate calculator — root-tab and soil-bed planning
Frequently Asked Questions (Aquarium Fertilizer)
How do I dose to a target ppm?
For a liquid product: dose_ml = (target_ppm × volume_L) / product_ppm_per_ml. For a dry salt:
mass_mg = (target_ppm × volume_L) / element_fraction. Example: to raise NO₃⁻ by 10 ppm in
40 gal using KNO₃ (61.3% NO₃⁻), you need about 2.47 g of KNO₃.
How often should I fertilize my aquarium?
Depends on the method. EI: 3× per week macros + 2× per week traces, with 50% weekly water change. PPS-Pro: daily small doses with smaller water changes. All-in-one products: follow the label schedule. Consistency matters more than any single dose.
What's the difference between EI and PPS-Pro?
EI (Estimative Index) doses high macros and traces with weekly 50% water changes to prevent any deficiency; it suits high-light, high-CO₂ fast-growth tanks. PPS-Pro doses lower, more precise amounts with smaller changes; it suits stability-focused, lower-light tanks. Both work; mixed or ad-hoc dosing is the common failure mode.
Can I overdose fertilizer in my aquarium?
Yes. Trace-element overdose is the bigger risk — especially copper and zinc, which are toxic to invertebrates at low levels. Macro overdose rarely harms fish directly, but shifts nutrient balance and can fuel algae if light/CO₂ are not in sync. Test and dose to target ppm, not "a little extra."
What happens if I don't fertilize my aquarium plants?
Depends on plant species and setup. Easy, slow-growing plants in a Walstad-style soil tank often do fine with no liquid fertilizer. Fast-growing stems in an inert substrate (gravel) will show deficiencies within weeks — yellowing, slow growth, or stunted new leaves.
How do I know if my plants need more fertilizer?
Look at the pattern: old leaves yellow → likely N; new leaves yellow with green veins → likely Fe; old leaves yellow between veins → likely Mg; holes in leaves → possible K deficiency or snail damage. Confirm with a test kit before dosing — many deficiencies look similar.
How do I identify a nutrient deficiency?
Use the "Symptom-to-Nutrient Map" table above, and test nitrate, phosphate, iron, and GH where possible. Note that uniform paleness or slow growth often indicates insufficient light or CO₂, not a nutrient issue. See the CO2 calculator and light wattage calculator.
Does excess fertilizer cause algae?
Not by itself. Modern practice points to light–CO₂–nutrient balance as the dominant factor: too much light relative to CO₂ and nutrients, or unstable CO₂, drive algae more than high nutrient levels do. EI tanks run high nutrients deliberately and stay algae-free when the other inputs are managed.
Is this calculator a substitute for observing my plants?
No. The calculator converts target ppm to a dose. Plant response (color, growth rate, algae) is the ultimate signal — adjust from observation and test data, not from a single number. For a full dosing plan, follow a published method (EI or PPS-Pro). Related tools: CO2 calculator, light wattage calculator, dosage calculator.
Sources and references
- Barr, T. — Estimative Index (EI) dosing methodology
- Edward — Perpetual Preservation System (PPS-Pro)
- Walstad, D. — Ecology of the Planted Aquarium
- Seachem, Aquarium Co-Op, NilocG — product labels and dosing guidance
- APHA/ASTM — Standard Methods for the Examination of Water and Wastewater