Urea as a Nitrogen Fertilizer in the Aquarium
Urea is the only potassium-free nitrogen source among the common DIY salts — useful when potassium is already sufficient. Unlike potassium nitrate, though, the nitrogen isn't immediately available as nitrate.
How urea actually works in the tank
Plants can't use urea directly. Bacteria in the tank first split it via an enzyme (urease) into ammonium/ammonia, which is then converted further to nitrite and nitrate by nitrifying bacteria — the same pathway fish waste takes. Urea nitrogen therefore passes through an intermediate stage that potassium nitrate skips entirely.
Is that dangerous?
Ammonia (NH₃) is toxic to fish; ammonium (NH₄⁺) much less so. Which fraction is present depends strongly on pH: at the 6.5–7 pH typical for CO₂-injected planted tanks, the toxic NH₃ fraction stays under 1% of total ammonia — above pH 8 it rises quickly. Standard ammonia test kits always measure total ammonia (NH₃+NH₄⁺ combined), not the toxic fraction alone — a spike on the test looks more alarming at low pH than it actually is.
Why the ammonia reading can still rise briefly
The tank's nitrifying bacteria are tuned to the usual, steady load from fish waste. A larger single dose of urea creates an ammonium surge that the bacterial population needs time to adjust to — a matter of days, not weeks. During that window the test reads elevated without automatically meaning danger, as long as pH stays in the usual range.
Practical dosing
Ease into it rather than giving the full target amount at once — small doses, increased over a few days. At higher pH (above roughly 7.5), dose more cautiously or switch to a different nitrogen source entirely. Agricultural prilled urea is often coated with a urease inhibitor (NBPT) that deliberately delays the conversion — unsuitable for aquarium dosing; use uncoated urea instead.
For anyone dosing by the numbers
1 g of urea delivers the same nitrogen as about 2.06 g of nitrate (NO₃⁻) — its high nitrogen content (46.65%) makes urea especially concentrated. A nitrate test won't catch this nitrogen right away, though: it only measures the nitrate that forms after bacterial conversion, not the urea itself.
Frequently Asked Questions
Is urea safe to use as a nitrogen fertilizer in the aquarium?
Yes, at the pH of 6.5–7 typical for CO2-injected planted tanks, combined with slow dosing. The toxic ammonia fraction stays under 1% of total ammonia there.
Why does my ammonia test spike briefly after dosing urea?
The tank's nitrifying bacteria need a few days to adjust to the higher ammonium load. During that time the test reads elevated, without automatically meaning danger at typical pH.
How does pH affect the toxicity of ammonia?
The toxic NH3 fraction of total ammonia rises with pH: under 1% at 6.5–7, already over 5% at 8.0, and over 15% at 8.5.
How do I dose urea correctly?
Ease into it instead of giving the full target amount at once, dose more cautiously at higher pH (above roughly 7.5), and avoid agricultural prilled urea coated with a urease inhibitor (NBPT).
Why doesn't my nitrate test pick up the urea right away?
The test measures nitrate, which only forms after bacterial conversion through ammonium and nitrite — not the urea itself.
The Sodachi calculator can plan urea as a nitrogen source precisely.