From your test kit to elemental numbers.
Reagent kits report compounds like nitrate (NO3) and phosphate (PO4). The GEN method thinks in elemental terms, so Ilaequor converts. Nitrate is mostly oxygen by weight, so the raw nitrate number overstates how much actual nitrogen (N) is present.
| Convert | Factor | Note |
|---|---|---|
| NO3 (nitrate) to elemental N (nitrogen) | divide by about 4.43 | from the molar mass ratio 62 / 14 = 4.428 |
| PO4 (phosphate) to elemental P (phosphorus) | divide by about 3.06 | from the molar mass ratio 95 / 31 = 3.064 |
| NH4 (ammonium) to NO3 equivalent | multiply by 3.65 | nitrification equivalent from the source calculator |
| Urea-N to NO3 equivalent | multiply by about 7.9 | source calculator factor; treat as approximate |
The ratios GEN cares about.
GEN manages a balance, not a set of absolute levels. These are GEN targets, not universal laws; where you sit inside them depends on plant load and CO2.
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Elemental N:K, about 1.4 to 2 : 1.
The ratio of elemental nitrogen to potassium (N to K). N here is elemental nitrogen, that is NO3 divided by 4.43, not raw nitrate. When prepping for a water change, the target sits nearer 1.3 to 1.6.
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Redfield NO3:PO4, about 10:1 to 10:1.5.
The nitrate-to-phosphate band for planted substrate. Phosphate is kept deliberately low as a growth limiter.
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Potassium is modeled, not tested.
GEN treats hobby potassium tests as unreliable, so K is usually tracked as a modeled value (what you dosed, diluted by water changes) rather than a tested reading.
GH, KH, and RO water.
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GH (general hardness).
The dissolved calcium and magnesium in your water.
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KH (carbonate hardness).
The carbonates and bicarbonates that buffer pH.
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RO (reverse osmosis) water.
Starts near zero GH and KH. You remineralize it to your target, which is where calcium and magnesium dosing comes in.
Which salts raise which elements.
The common dry salts and the elements they raise. Mass fraction is the share of the salt's weight that is the element you care about.
| Salt | Raises | Approximate element mass fraction |
|---|---|---|
| KNO3 (potassium nitrate) | NO3 and K | NO3 about 0.613, K about 0.387 |
| KH2PO4 (monopotassium phosphate) | PO4 and K | PO4 about 0.698, K about 0.287 |
| K2SO4 (potassium sulfate) | K | K about 0.449 |
| KCl (potassium chloride) | K | K about 0.524 |
| MgSO4 7H2O (Epsom salt) | Mg | Mg about 0.099; also raises GH |
| CaSO4 (gypsum) | Ca | Ca about 0.276; raises GH, not KH |
| CaCO3 (calcium carbonate) | Ca | Ca about 0.401; raises KH and GH |
One salt can raise several elements at once (KNO3 adds both nitrate and potassium), which is exactly why single-number dosing gets fiddly by hand. Ilaequor works from a fuller registry of about 25 salts, with GH and KH side-effect flags, inside the app.
The math behind each dose.
For a stock of a given salt (some grams of it dissolved into a volume of water, stock_ml) dosed into a tank of a known volume (tank_L, in litres), one millilitre of that stock raises the tank by:
delta_mgL_per_ml = grams * 1000 / stock_ml / tank_L * element_mass_fractionelement_mass_fraction is the share of the salt's weight that is the element you want (from the table above). One salt can feed several elements, so the formula runs once per element using that salt's fraction each time. Invert it and you get the recommendation: how many millilitres to add to raise an element by a target amount. This is the same arithmetic as a dosing spreadsheet; Ilaequor just runs it for you from your own stocks and tank volume.
A note on the numbers.
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The urea factor is approximate.
The urea-N to NO3 equivalent (about 7.9) comes from the source calculator; treat it as approximate.
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Ranges are starting points.
Any typical range beyond the GEN bands above is a starting point, not a rule. Real numbers vary widely with your setup, plant load, and CO2.
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These are the numbers Ilaequor runs.
The app applies exactly these conversions, ratios, and fractions so you do not have to keep them in your head or your spreadsheet.
Want the app that runs these numbers?
Ilaequor times your tests, models the tank between them, and turns each session into next-period doses from your own fertilizers. It is in active development. Join the list for updates and which platform launches first.
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