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20 US gal · 75.7 L

Fertilizer calculator for a 20 gallon planted tank

Exact doses for a 20 US gallon tank (75.7 liters), computed with the same engine as the dosing calculator. Open it prefilled to change light level, water change or substrate volume.

Open the 20 gal EI calculator20 gal stock solution

Dry dosing: grams per dose for 20 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 75.7 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)0.926 g (926 mg)0.494 g (494 mg)0.617 g (617 mg)
KH₂PO₄ (monopotassium phosphate)0.108 g (108 mg)0.054 g (54.2 mg)0.054 g (54.2 mg)
K₂SO₄ (potassium sulfate)0.82 g (820 mg)0.383 g (383 mg)0.277 g (277 mg)
MgSO₄·7H₂O (Epsom salt)1.54 g0.768 g (768 mg)0.768 g (768 mg)
CSM+B (trace mix)0.232 g (232 mg)0.116 g (116 mg)0.116 g (116 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 20 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)11.15 g KNO₃ + 1.31 g KH₂PO₄ + 9.88 g K₂SO₄ + 18.5 g MgSO₄·7H₂O
Micro (EI)2.79 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 20 gallons

ProductLabel dose for 20 galHow often (label)
Seachem Flourish1.5 mL5 mL per 250 L (60 US gal) once or twice a week
Seachem Flourish Nitrogen1.2 mL2.5 mL per 160 L (40 US gal) twice a week
Seachem Flourish Phosphorus2.4 mL2.5 mL per 80 L (20 US gal) once or twice a week
Seachem Flourish Potassium3 mL5 mL per 125 L (30 US gal), 2–3 times a week
Tropica Specialised Nutrition9.1 mL6 mL (3 pumps) per 50 L once a week at the water change
Tropica Premium Nutrition9.1 mL6 mL (3 pumps) per 50 L once a week at the water change

Scaled linearly from each maker's directions. Seachem Flourish at 1.5 mL adds about 0.064 ppm iron; Tropica Specialised at 9.1 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 20 gallon tank?

For EI, about 0.926 g (926 mg) per dose, three times a week (22.5 ppm NO₃ a week). Lean EI is about 0.494 g (494 mg) per dose. A low-tech tank needs about 0.617 g (617 mg) once a week.

How much KH2PO4 for a 20 gallon planted tank?

About 0.108 g (108 mg) per dose for EI (3 ppm PO₄ a week over three doses) and 0.054 g (54.2 mg) for lean EI.

How much Seachem Flourish for a 20 gallon tank?

Seachem's directions are 5 mL per 250 L (60 US gal) once or twice a week, which is about 1.5 mL for 20 gallons. Each cap thread is about 1 mL.

How much Tropica Specialised Nutrition for 20 gallons?

Tropica recommends 6 mL (3 pumps) per 50 L each week at the water change, so about 9.1 mL (4.5 pumps) a week for 76 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 gallon29 gallon40 gallon55 gallon75 gallon125 gallon

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