# Aquarium nitrogen cycle tracker

> Enter your ammonia, nitrite, and nitrate readings in ppm (parts per million) from a standard test kit. A reading of 0 ppm ammonia, 0 ppm nitrite, and 20 ppm nitrate is "cycling" — the worked example below uses the same inputs so the verdict shown here always matches the calculator.

Use the interactive tracker on the page: https://petguidecalc.com/aquarium/nitrogen-cycle/. Enter your ammonia (NH₃ / NH₄⁺), nitrite (NO₂⁻), and nitrate (NO₃⁻) readings in ppm and the verdict tells you whether the cycle is unsafe, cycling, established, or overdue for a water change.

## What do my ammonia, nitrite, and nitrate readings mean?

Ammonia and nitrite above 0 ppm are always UNSAFE — both are toxic at any measurable level. Nitrate between 0 and 40 ppm with zero ammonia / nitrite means the biofilter is producing some nitrate but the colony hasn't fully matured yet (CYCLING). Nitrate at or above 40 ppm with zero ammonia / nitrite means the cycle is ESTABLISHED, and you should do a partial water change to bring nitrate back below 40 ppm.

## Reading combinations at a glance

| Scenario | NH₃ ppm | NO₂⁻ ppm | NO₃⁻ ppm | Verdict |
|---|---|---|---|---|
| Just-set-up tank | 0 | 0 | 0 | UNSAFE |
| Mid-cycle | 0 | 0 | 20 | CYCLING |
| Established, due for change | 0 | 0 | 50 | ESTABLISHED |
| Ammonia spike | 0.5 | 0 | 10 | UNSAFE |
| Stalled biofilter | 0 | 1 | 5 | UNSAFE |

Ammonia and nitrite above 0 ppm are always UNSAFE. Nitrate at or above 40 ppm with zero ammonia/nitrite triggers ESTABLISHED; well above 40 ppm triggers WATER-CHANGE-DUE.

## How to use this tracker

1. **Test the water** — Use a standard aquarium test kit to measure total ammonia, nitrite, and nitrate in ppm from the same water sample.

2. **Enter all three readings** — Enter the ammonia, nitrite, and nitrate values in the tracker. Use 0 when the test kit reads zero and do not leave a field blank.

3. **Read the cycle status** — Treat any measurable ammonia or nitrite as UNSAFE. With both at zero, nitrate at 0 means the cycle has not established, nitrate above 0 and below 40 ppm is CYCLING, and nitrate from 40 to below 80 ppm is ESTABLISHED.

4. **Take the next action** — For UNSAFE readings, do a partial water change, cut feeding, and retest in 24 hours. For ESTABLISHED readings at or above 40 ppm, change water to bring nitrate below 40 ppm; 80 ppm or higher is WATER-CHANGE-DUE.

## How the verdict is calculated

Ammonia or nitrite above 0 ppm ⇒ UNSAFE. Both are toxic to fish at any measurable level. Do a partial water change, cut feeding, and retest in 24 hours.

All three readings at 0 ppm ⇒ UNSAFE. The biofilter has not produced any nitrate yet, so the cycle has not built up. Continue fishless cycling or seed the filter with established media.

Nitrate above 0 but below 40 ppm with zero ammonia/nitrite ⇒ CYCLING. The biofilter is producing some nitrate but the colony has not fully matured. Keep monitoring weekly.

Nitrate at or above 40 ppm with zero ammonia/nitrite ⇒ ESTABLISHED. The cycle is fully built up; do a partial water change to bring nitrate back below 40 ppm.

Nitrate well above 40 ppm ⇒ WATER-CHANGE-DUE. The cycle is established but the tank needs a large partial water change (or a series of smaller changes) to bring nitrate back to a safe range.

Worked example: 0 / 0 / 20 ppm reads "cycling". Change any input and the verdict updates from the same shared logic.

## Nitrogen cycle questions, answered

**Q: What is the aquarium nitrogen cycle?**

The nitrogen cycle is the biofilter that converts toxic ammonia (NH₃ / NH₄⁺) into less harmful nitrate (NO₃⁻) via nitrite (NO₂⁻). It runs in every healthy aquarium and is what makes fish-in cycling survivable.

**Q: What ammonia and nitrite levels are safe?**

Zero. Both are toxic at any measurable level. Even 0.25 ppm ammonia stresses fish and damages gill tissue; nitrite binds hemoglobin and blocks oxygen uptake. The cycle is only "established" when both stay at 0 ppm for weeks at a time.

**Q: When is nitrate too high?**

Keep nitrate below 40 ppm for freshwater community tanks and below 20 ppm for planted or sensitive tanks. Anything above 80 ppm is overdue for a partial water change.

**Q: How long does the cycle take to establish?**

A fishless cycle using pure ammonia typically takes 3–6 weeks. A seeded cycle (established media from another tank) can complete in 1–2 weeks. The cycle is established when ammonia and nitrite stay at 0 ppm and nitrate has built up to about 40 ppm consistently.

**Q: My readings are all 0 ppm — what does that mean?**

It usually means the cycle has not started yet (common on day one or after a full reset). The biofilter has not produced any nitrate yet, so the colony needs to build up. Keep fishless-cycling with ammonia or seed the filter with established media, then retest in a few days.

## References

The verdict thresholds come from mainstream aquarium hobby sources and the API test kit documentation; the calculator applies those exact thresholds.

- [A nitrogen removal system to limit water exchange for recirculating freshwater aquarium using DHS-USB reactor](https://doi.org/10.1080/09593330.2017.1333530) — Adlin N, Matsuura N, Ohta Y, Hirakata Y, Maki S, Hatamoto M, et al., Environ Technol, 2018.
- [The aquarium nitrogen cycle](https://www.aquariumcoop.com/) — Aquarium Co-Op, Aquarium Co-Op. Hobby reference for the ammonia → nitrite → nitrate chain and the safe vs. unsafe thresholds used by the calculator.
- [Fishless cycling and ammonia toxicity](https://www.tfhmagazine.com/) — TFH Magazine, TFH Magazine. Long-running aquarium publication confirming the 0 ppm ammonia / 0 ppm nitrite targets and the partial-water-change cadence to keep nitrate in check.
- [API test kit thresholds](https://apifishcare.com/) — API (Mars Fishcare), API. Test kit manufacturer reference for the ammonia / nitrite / nitrate ranges the calculator applies as its verdict boundaries.

_Educational resource, not veterinary advice. PetGuideCalc — https://petguidecalc.com_