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125 US gal · 473.2 L

Fertilizer calculator for a 125 gallon planted tank

Exact doses for a 125 US gallon tank (473.2 liters), computed with the same engine as the dosing calculator. Open it prefilled to change light level, water change or substrate volume.

Open the 125 gal EI calculator125 gal stock solution

Dry dosing: grams per dose for 125 gallons

Weigh each salt on a 0.01 g scale and add it straight to the tank. EI and lean EI split the weekly amount into 3 macro doses and 3 micro doses on alternate days; low-tech is one dose a week. Numbers assume the full 473.2 L; if substrate and rock take 15 % of the space, multiply by 0.85.

SaltEI (per dose, 3×/wk)Lean EI (per dose, 3×/wk)Low-tech (once a week)
KNO₃ (potassium nitrate)5.79 g3.09 g3.86 g
KH₂PO₄ (monopotassium phosphate)0.678 g (678 mg)0.339 g (339 mg)0.339 g (339 mg)
K₂SO₄ (potassium sulfate)5.12 g2.4 g1.73 g
MgSO₄·7H₂O (Epsom salt)9.6 g4.8 g4.8 g
CSM+B (trace mix)1.45 g0.725 g (725 mg)0.725 g (725 mg)

Weekly targets used (ppm added per week): EI NO₃ 22.5, PO₄ 3, K 30, Mg 6, Fe 0.6; lean EI about half; low-tech NO₃ 5, PO₄ 0.5, K 5. K₂SO₄ only tops up potassium that KNO₃ and KH₂PO₄ don't already supply. The CSM+B amount assumes a typical 6.53 % iron label; check your bag.

With EI and a 50 % weekly water change, nitrate settles at about 45 ppm just before each change (weekly dose ÷ 0.5). Change less water and it climbs higher; see how water changes affect dosing.

Stock solution recipe for a 125 gallon tank (500 mL bottles)

Dissolve in distilled or RO water, top up to 500 mL, and dose 42 mL per dose (the bottle lasts about 4 weeks at 3 doses a week). Keep macros and micros in separate bottles.

BottleDissolve in 500 mL
Macro (EI)69.72 g KNO₃ + 8.17 g KH₂PO₄ + 61.73 g K₂SO₄ + 115.62 g MgSO₄·7H₂O
Micro (EI)17.46 g CSM+B

Warning: K2SO4 is close to its solubility limit at this strength; use a bigger bottle or a larger dose volume. Some keepers move MgSO₄ to the micro bottle or dose it dry; see where MgSO₄ goes and how to make a stock solution.

Liquid fertilizers: label doses for 125 gallons

ProductLabel dose for 125 galHow often (label)
Seachem Flourish9.5 mL5 mL per 250 L (60 US gal) once or twice a week
Seachem Flourish Nitrogen7.4 mL2.5 mL per 160 L (40 US gal) twice a week
Seachem Flourish Phosphorus15 mL2.5 mL per 80 L (20 US gal) once or twice a week
Seachem Flourish Potassium19 mL5 mL per 125 L (30 US gal), 2–3 times a week
Tropica Specialised Nutrition57 mL6 mL (3 pumps) per 50 L once a week at the water change
Tropica Premium Nutrition57 mL6 mL (3 pumps) per 50 L once a week at the water change

Scaled linearly from each maker's directions. Seachem Flourish at 9.5 mL adds about 0.064 ppm iron; Tropica Specialised at 57 mL a week adds about 6.91 ppm nitrate-equivalent. Compare products in the product library.

FAQ

How much KNO3 do I dose in a 125 gallon tank?

For EI, about 5.79 g per dose, three times a week (22.5 ppm NO₃ a week). Lean EI is about 3.09 g per dose. A low-tech tank needs about 3.86 g once a week.

How much KH2PO4 for a 125 gallon planted tank?

About 0.678 g (678 mg) per dose for EI (3 ppm PO₄ a week over three doses) and 0.339 g (339 mg) for lean EI.

How much Seachem Flourish for a 125 gallon tank?

Seachem's directions are 5 mL per 250 L (60 US gal) once or twice a week, which is about 9.5 mL for 125 gallons. Each cap thread is about 1 mL.

How much Tropica Specialised Nutrition for 125 gallons?

Tropica recommends 6 mL (3 pumps) per 50 L each week at the water change, so about 57 mL (28.4 pumps) a week for 473 L.

Should I dose for the full tank volume?

Dose for the water actually in the tank. Substrate, rock and wood often take 10 to 25 % of the nominal volume, so many people dose for about 85 %. Under-dosing slightly is safer than over-dosing, especially with shrimp.

Other tank sizes

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