TankDosing
EN
40 US gal · 151.4 L

Fertilizer calculator for a 40 gallon planted tank

Exact doses for a 40 US gallon tank (151.4 liters), computed with the same engine as the dosing calculator. Open it prefilled to change light level, water change or substrate volume.

Open the 40 gal EI calculator40 gal stock solution

Dry dosing: grams per dose for 40 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 151.4 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)1.85 g0.988 g (988 mg)1.23 g
KH₂PO₄ (monopotassium phosphate)0.217 g (217 mg)0.108 g (108 mg)0.108 g (108 mg)
K₂SO₄ (potassium sulfate)1.64 g0.767 g (767 mg)0.554 g (554 mg)
MgSO₄·7H₂O (Epsom salt)3.07 g1.54 g1.54 g
CSM+B (trace mix)0.464 g (464 mg)0.232 g (232 mg)0.232 g (232 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 40 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)22.31 g KNO₃ + 2.61 g KH₂PO₄ + 19.75 g K₂SO₄ + 37 g MgSO₄·7H₂O
Micro (EI)5.59 g CSM+B

All salts stay well under their solubility limits at this strength. 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 40 gallons

ProductLabel dose for 40 galHow often (label)
Seachem Flourish3 mL5 mL per 250 L (60 US gal) once or twice a week
Seachem Flourish Nitrogen2.4 mL2.5 mL per 160 L (40 US gal) twice a week
Seachem Flourish Phosphorus4.7 mL2.5 mL per 80 L (20 US gal) once or twice a week
Seachem Flourish Potassium6.1 mL5 mL per 125 L (30 US gal), 2–3 times a week
Tropica Specialised Nutrition18 mL6 mL (3 pumps) per 50 L once a week at the water change
Tropica Premium Nutrition18 mL6 mL (3 pumps) per 50 L once a week at the water change

Scaled linearly from each maker's directions. Seachem Flourish at 3 mL adds about 0.064 ppm iron; Tropica Specialised at 18 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 40 gallon tank?

For EI, about 1.85 g per dose, three times a week (22.5 ppm NO₃ a week). Lean EI is about 0.988 g (988 mg) per dose. A low-tech tank needs about 1.23 g once a week.

How much KH2PO4 for a 40 gallon planted tank?

About 0.217 g (217 mg) per dose for EI (3 ppm PO₄ a week over three doses) and 0.108 g (108 mg) for lean EI.

How much Seachem Flourish for a 40 gallon tank?

Seachem's directions are 5 mL per 250 L (60 US gal) once or twice a week, which is about 3 mL for 40 gallons. Each cap thread is about 1 mL.

How much Tropica Specialised Nutrition for 40 gallons?

Tropica recommends 6 mL (3 pumps) per 50 L each week at the water change, so about 18 mL (9.1 pumps) a week for 151 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

10 gallon20 gallon29 gallon55 gallon75 gallon125 gallon

Sources