Calculator
Nitrate and water change calculator
With regular water changes nitrate doesn't keep rising, it settles at a steady level. The calculator shows that level and the water change you need to hit your target.
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Where nitrate comes from and how to measure it
Nitrate is the last stage of the nitrogen cycle: what is left after food and waste break down. Measure the weekly increase with two NO3 tests: right after a water change and a week later, just before the next one.
Nitrate and water change calculator
The steady-state nitrate formula
NO3 before a change ≈ weekly increase ÷ fraction of water changed
Example: an increase of 10 mg/l a week with 30% changes (0.3) gives about 33 mg/l before a change and 23 mg/l after it. With 50% changes it is 20 and 10 mg/l.
What nitrate level is good
| NO3 | Verdict |
|---|---|
| 0–5 mg/l (ppm) | too low in a planted tank: plants starve, cyanobacteria get easier |
| 10–25 mg/l | a good range for most fish and plants |
| 25–50 mg/l | acceptable for hardy fish, worth increasing water changes |
| above 50 mg/l | harmful to fish and feeds algae |
How to slow the nitrate rise
- Feed what the fish eat in 2 minutes.
- Siphon the substrate during water changes and rinse filter media in tank water.
- Add fast-growing plants that take up nitrate.
- Test your tap water: if it contains nitrate, water changes lower it less.
More in the guide how often to change aquarium water. Aquarium Monitor records NO3 test results and shows the trend between water changes.
Frequently asked questions
How much water should I change when nitrate is high?
Enough that the weekly increase divided by the fraction changed is below your target. With an increase of 15 mg/l a week and a 25 mg/l target that is at least 60% weekly, or two 30% changes.
Why didn't nitrate drop as much as I expected after a water change?
Most often the tap water contains nitrate too. Test it and subtract it from the result.
Keep an eye on your tank from your phone
The app is free and ad-free. The monitor runs on your Raspberry Pi.