Aquarium Fertilizer Dosing Calculator (EI, PPS-Pro, Custom)

How much KNO₃, KH₂PO₄ and K₂SO₄ per dose for EI?

For a full Estimative Index dose — 7.5 ppm nitrate, 1.3 ppm phosphate, 7.5 ppm potassium, 1 ppm magnesium and 0.2 ppm iron, three times a week — 100 liters takes 1.22 g of potassium nitrate, 0.19 g of monopotassium phosphate, 0.5 g of potassium sulfate, 1.01 g of Epsom salt and 0.29 g of CSM+B. Every one of those is a molar mass: KNO₃ is 61.3 % nitrate by weight, so 7.5 mg of nitrate per liter is 7.5 ÷ 0.613 mg of the salt per liter. Teaspoons are printed at stated densities and are ±20 % by grain size; a $15 scale that reads to 0.01 g is the better tool. The chart below is one dose by tank size.

Do I need K₂SO₄ at all? Potassium is already in the other two salts

Potassium nitrate is 39 % potassium and monopotassium phosphate is 29 %, so the nitrate and phosphate doses bring potassium with them whether you wanted it or not. At the standard EI dose that is 4.7 ppm K from the KNO₃ and 0.5 from the KH₂PO₄ — 5.3 of the 7.5 ppm target before any potassium sulfate goes in. K₂SO₄ is a top-up of 2.2 ppm, not a third full dose. Size it to the whole target — which is what happens when a chart lists "K₂SO₄: K 7.5 ppm" as its own line — and the dose delivers 12.8 ppm, 70 % over. Harmless, since potassium is about the most forgiving nutrient there is, but it is the reason two calculators give different K₂SO₄ answers for the same tank, and at high nitrate targets the tool will tell you K₂SO₄ has dropped to zero because KNO₃ alone already covers the potassium.

Potassium by mass fraction of each salt. The K target is total potassium; PPS-Pro's 1.33 ppm is stated that way by its author.

How high does EI actually run? The 50 % water change math

"Estimative" means you overshoot on purpose and let the water change set the ceiling. The ceiling is arithmetic: dose D per week between changes of a fraction w, and the level just before the change settles at D ÷ w if the plants take nothing. At 50 % that is twice the weekly dose — EI's 22.5 ppm of nitrate a week settles at 45 ppm before the change and 23 after, reaching 90 % of that in 3.3 weeks. Real tanks run lower because the plants do take some; the ceiling is what you get if they don't, which is exactly the case (new tank, melted plants, CO₂ off) where people wonder why nitrate is 40. Two things follow. First, the water change is not optional: skip it and there is no ceiling at all. Second, that peak is above the 20 ppm nitrate line on the freshwater parameters chart — by design. Nitrate at 40 ppm is not what harms fish in a planted tank; the CO₂ is, and the water change calculator runs the same arithmetic for a tank that is producing nitrate rather than being dosed with it.

22.5 ppm NO₃ dosed per week, zero uptake — the ceiling. Every other nutrient scales the same way: PO₄ settles at 7.8 ppm and K at 45 at 50 %.

How do I make a fertilizer stock solution?

Multiply one dose by the number of doses in the bottle. For the 100 L tank above, 500 mL at 10 mL a dose is 50 doses: 61.1 g KNO₃, 9.3 g KH₂PO₄, 24.9 g K₂SO₄ and 50.7 g Epsom salt in one macro bottle, and 14.3 g CSM+B in a separate one — iron chelate and phosphate in the same bottle precipitate, which is why every published regime keeps two. The limit is solubility, and potassium sulfate hits it first: a 250 mL bottle at 5 mL a dose for a 600 L tank needs 598 g/L of K₂SO₄ against a ceiling of 111 — it will not dissolve, and the doses from that bottle are wrong in a way you can't see. The tool checks every line; use a bigger bottle or a bigger dose.

Solubility at about 20 °C (handbook figures); salts sharing a bottle dissolve a little less than alone, so treat "near the limit" as over it. Warm RO water, and let it cool before dosing.

ppm per week; "settles at" is the zero-uptake peak before the weekly change. Sources on each preset in the tool's math panel.

How much fertilizer for a low-light or low-tech tank?

Less, and the reason is the accumulation math above. Without injected CO₂, plants grow at a fraction of the rate and take a fraction of the nutrients, so the ceiling is what you actually see. Barr's own advice for low light is a half to a quarter of full EI; a quarter dose for 100 L is 0.31 g of KNO₃ and 0.05 g of KH₂PO₄, which is small enough that a weekly single dose is the sane form — set the tool to one dose a week with the per-dose numbers at a quarter. Fish food supplies a good deal of nitrate and phosphate in a stocked low-tech tank, and many run on that plus traces and potassium alone. Test nitrate before adding any: if the tank makes its own, the fertilizer to add is the one it can't.

I dose EI and still have algae or deficiencies — what's wrong?

Almost never the fertilizer, which is the uncomfortable part of a method built on excess. In an injected tank the limiting factor is nearly always CO₂ — not the ppm at noon, but the stability from one day to the next, which is what the CO₂ calculator is for and what the algae ID guide ties to black beard algae. Second is the water itself: magnesium and calcium come from GH, not from the macro bottle, and RO-water tanks dosing perfect EI into 1 dGH water show "deficiencies" the GH / KH adjustment calculator fixes in a bucket. Third is iron: chelated iron precipitates with phosphate and breaks down under strong light, so a trace dose on the same day as the macros, or a single weekly one, delivers less than the arithmetic says — hence the alternating days. What the fertilizer arithmetic can rule out is the thing people reach for first: more. The EI explained guide covers the reasoning; this page owns the numbers.