Water tests and fertilizers: N, P and CO₂
Tests are not for a tidy report. They show what is missing and what is piling up, and then the ratios start to matter.
What the tests measure
Fifteen parameters fall into three groups: water (pH, KH, GH, TDS, EC), nutrition (NO₃, PO₄, K, Ca, Mg, Fe) and control (NH₃/NH₄, NO₂, Cu). Three groups, three different questions: what the water is, what the plants have to eat, and what is dangerous right now.
An empty field means "no test". Zero is never substituted for it, because that would make the calculation report a deficiency you do not actually have.
Nitrogen and phosphorus: what accumulates
In food the ratio is roughly NO₃ : PO₄ = 5.8 : 1. Plants want about 10 : 1, or 7 : 1 by element for N : P. Nitrogen runs out first, phosphate keeps building up. That is how algae appear in tanks where every test looks normal.
A water change lowers both by the same percentage and leaves the ratio untouched. To get rid of phosphate you need plant growth running on nitrogen, or selective binding, or more frequent changes combined with tighter control of feeding.
The ratio is what gets lost when people stare at single numbers. Nitrate is fine, phosphate is high, and the tank still grows algae.
CO₂ from pH and KH
To estimate carbon dioxide: CO₂ (mg/L) ≈ 3 × KH × 10^(7 − pH). At KH 4 and pH 6.8 the result is about 19 mg/L, a working range for a planted tank.
The method breaks down when other buffers are present: phosphates, humates, RO water with a remineralizer. Then pH says nothing about CO₂, and you need a direct test or a pH controller instead.
Three salts for nitrogen
| Salt | Adds | Limit |
|---|---|---|
| Potassium nitrate, KNO₃ | nitrogen + potassium | hits the potassium ceiling |
| Calcium nitrate, Ca(NO₃)₂ | nitrogen + calcium | hits the calcium ceiling |
| Urea, CO(NH₂)₂ | nitrogen only | partly turns into ammonia at high temperature and pH |
The calculator doses the nitrates up to the potassium and calcium limits and covers the remainder with urea. At potassium 15 almost all the nitrogen goes in as potassium nitrate; at 30 that salt is ruled out completely.
Why the salts do not move pH
Nitrate and phosphate, at the doses we add, do not shift pH noticeably: the buffering capacity of the water is orders of magnitude larger. pH moves with CO₂ and carbonates, not with fertilizers. A few milligrams per litre cannot compete with carbonate hardness measured in degrees, so acidifying water with fertilizers does not work.
In practice
- Test once a week, at the same time of day, before the water change.
- Do not mix phosphate with calcium, magnesium or iron: the phosphates of those metals precipitate.
- Store the solutions separately, iron and phosphate above all.
- Above 25 °C and pH 7.5, give urea as a fraction of the nitrogen.
The point of the tests is not a tidy report. They show what is missing and what is piling up, and from there it is arithmetic on ratios rather than guesswork.
Calculator
The calculator takes your dimensions and works out glass, weight, stand and doses.
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