mg/L
Nitrate
What is it
Nitrate (NO₃⁻) is the final product of the nitrogen cycle: fish ammonia is converted to nitrite and then to nitrate by nitrifying filter bacteria. Unlike ammonia and nitrite, nitrate is relatively low in toxicity and plants consume it as a macronutrient.
In a planted aquarium, nitrate has a dual role: it is both a pollution indicator when too high and an essential fertiliser when too low. Finding the right balance is one of the central challenges of aquarium care.
The main nitrate sources are the biological nitrogen cycle and organic decomposition, tap water (which can contain 10-40 mg/L in agricultural areas), and direct KNO₃ fertilisation in high-demand planted tanks.
Why it matters
For plants, nitrate is the preferred nitrogen form: without enough nitrogen they cannot make proteins, chlorophyll or new growth. Nitrate deficiency appears as progressively yellowing old leaves, unlike iron deficiency, which affects new leaves.
For fish, moderate nitrate levels (below 30 mg/L) are not dangerous, but chronic exposure to high levels (above 40-50 mg/L) causes stress, immune suppression, reduced fertility and susceptibility to disease.
Nitrate is also a main algae nutrient: when plants cannot consume it all because light, CO2 or another nutrient is lacking, algae benefit. A tank with 40 mg/L nitrate but little light and no CO2 can become an algae haven.
Ideal ranges
Interactions with other parameters
Nitrate must be balanced with phosphate, potassium, light and CO2, including the classic Redfield N:P ratio of 10:1 or 15:1. A severe nitrate shortage stops photosynthesis, leaving unused light that can trigger algae. Algae problems usually come from imbalance and instability rather than nitrate alone.
| Tank type | Min | Max | Unit |
|---|---|---|---|
| Tropical community | 5 | 30 | mg/L |
| Planted high-tech | 10 | 25 | mg/L |
| Planted low-tech | 5 | 20 | mg/L |
| Shrimp tank | 0 | 15 | mg/L |
Out of range: what happens
Nitrate that is too low (below 5 mg/L in planted tanks) causes slow growth, old leaves that yellow and fall, stunted stem plants, and possible cyanobacteria (BGA), which fix atmospheric nitrogen when nitrate is absent.
Moderately high nitrate (30-50 mg/L) causes no immediate fish problem, but increases algae risk, especially when other parameters are unbalanced. Shrimp begin to show stress.
Very high nitrate (above 50 mg/L) causes chronic stress in fish, faded colours, apathetic behaviour, susceptibility to Hole-in-the-Head in cichlids, and shrimp mortality. It indicates insufficient maintenance.
Common Myths
- Nitrate causes algae; algae thrive because of imbalance, often in CO2, rather than high nitrate itself. Planted aquariums usually need some available nitrogen, so nitrate must not always be zero.
- Water changes are optional if nitrate is low; they also reset many dissolved organics.
Measurement and adjustment
How to measure
A liquid reagent test (API Nitrate) is the most common method: shake reagent 2 vigorously for at least 30 seconds because it contains settling crystals. Without adequate shaking, the result will be falsely low.
Digital photometers (Hanna Checker) offer greater precision and are recommended for aquarists dosing nitrogen fertilisers. They cost more but give reliable results.
Test nitrate weekly as part of the normal routine and test tap water as well. In high-demand planted tanks, test 2-3 times per week to calibrate KNO₃ dosing.
How to adjust
To lower nitrate, use regular water changes, add fast-growing plants such as Limnophila, Hygrophila and floaters such as Salvinia and Pistia, reduce feeding load, remove detritus with a siphon, and stock sensibly.
To raise nitrate in planted tanks, dose potassium nitrate (KNO₃) according to the chosen fertilisation regime. EI uses 20-30 mg/L per week and PPS-PRO about 5-10 mg/L. Many all-in-one fertilisers already include nitrate.
For balance, maintain an NO₃:PO₄ ratio of about 10:1, for example 20 mg/L NO₃ and 2 mg/L PO₄. This is not a rigid rule, but it helps prevent deficiencies and algae.
Pro Tips
- In high-tech tanks, keep NO3 between 10 and 20 ppm. If it falls to zero, you risk an invasion of cyanobacteria (blue-green algae).
- If you keep fish that produce a high organic load but have few plants, use nitrate-specific filter media such as resins, floating plants or emergent pothos.