Nitrate (NO₃)
The end product of the nitrogen cycle. Plant fertiliser at the right level, fish stressor when it climbs.
What it is
Nitrate is the final, relatively harmless form of nitrogen that biological filtration produces after bacteria break down ammonia (NH₄) and nitrite (NO₂). It accumulates between water changes because nothing in a typical freshwater filter removes it — plants take some up as fertiliser, and a partial water change resets the rest.
Why it matters
For a planted tank, NO₃ is the headline macronutrient: too low and the plants stunt, too high and you'll see brown algae creeping over the slow growers. For a fish-only or shrimp tank, it's a stress indicator — sustained high NO₃ means the tank is producing more nitrogen than the water-change cadence is removing, which usually points to either too many fish or too few water changes.
How to test
Liquid drop tests (JBL NO₃, Tetra NO₃, Salifert) read in mg/L, and almost everyone uses these. Strip tests work in a pinch but lose accuracy fast above 25 mg/L. Test the day before a planned water change, so the number reflects the worst-case end of the cycle.
What high and low look like
Below 5 mg/L in a planted tank: plants pale, new leaves come in small, algae shifts toward green hair. Below 1 mg/L: sustained nutrient starvation, cyanobacteria can take hold. Above 50 mg/L: fish breathing harder, shrimp moulting irregularly. Above 100 mg/L: acute toxicity; do an emergency water change.
How to fix
Too low — dose a complete plant fertiliser containing nitrate (Tropica Specialised, Aqua Rebell Makro Basic NPK). The dosing calculator works out millilitres from your current and target numbers. Too high — water changes. A 30% change drops NO₃ by 30%; the water-change impact tool will show you exactly where you'll land.
