Fish Tank Nitrogen Cycle Calculator
Enter ammonia, nitrite, nitrate, pH and temperature to see your tank's cycle phase, the free (toxic) ammonia share and a water change suggestion. How it works
Use the Fish Tank Nitrogen Cycle Calculator
About the Fish Tank Nitrogen Cycle Calculator
A tank is cycled when its bacteria turn ammonia into nitrite and nitrite into nitrate fast enough that both read zero. This calculator reads one set of test results: ammonia below 0.05 ppm, nitrite below 0.1 ppm and some nitrate is Phase 4, the pattern of a cycled tank. It also shows how much of your ammonia is free NH3, the form that harms fish: 0.57% of the total at 77°F and pH 7.0, but 5.45% at pH 8.0.
Share of total ammonia that is free (un-ionized) NH3
| Temperature | pH 6.5 | pH 7.0 | pH 7.5 | pH 8.0 | pH 8.5 |
|---|---|---|---|---|---|
| 68°F (20°C) | 0.13% | 0.40% | 1.26% | 3.87% | 11.29% |
| 77°F (25°C) | 0.18% | 0.57% | 1.79% | 5.45% | 15.42% |
| 86°F (30°C) | 0.26% | 0.81% | 2.52% | 7.54% | 20.51% |
From the equation the calculator uses. Multiply your test-kit ammonia reading by the percentage: 1 ppm at 77°F and pH 8.0 is about 0.055 ppm of free NH3. Each step of 0.5 in pH roughly triples the free share.
How the readings map to a phase
| Phase | Ammonia | Nitrite | Nitrate | Days shown |
|---|---|---|---|---|
| 1: Ammonia spike | above 0.5 ppm | below 0.5 ppm | any | about 21 |
| 2: Nitrite spike | above 0 | above 0.5 ppm | any | about 14 |
| 3: Nitrate building up | below 0.1 ppm | below 0.5 ppm | above 5 ppm | about 7 |
| 4: Cycled pattern | below 0.05 ppm | below 0.1 ppm | above 0 | Done |
Checked in the order 1, 2, 4, 3. Readings that fit no row, such as 0 ammonia with 2 ppm nitrite or 0.3 ppm ammonia with 0.2 ppm nitrite, show no phase and no results.
Methodology
Free NH3 fraction = 1 ÷ (1 + 10^(0.09018 + 2727.8 ÷ T − pH)), with T in kelvin. The toxicity index is free NH3 ÷ 0.05 ppm in a fishless cycle or ÷ 0.02 ppm in a fish-in cycle, as a percentage capped at 100: under 20 is Safe, under 50 Moderate, under 80 High, and above that Dangerous. The phase comes from the table above. Nitrate above 40 ppm suggests a 20% water change and above 80 ppm a 50% change. The fishless dose for 2 ppm is 2 × litres ÷ (strength × 10) mL with the bottle’s strength entered, or a rough tank gallons × 2 ÷ 8 mL without it.
How to use this tool
- Choose Fishless Cycle or Fish-in Cycle.
- Enter ammonia (0 to 8 ppm), nitrite (0 to 10 ppm) and nitrate (0 to 160 ppm) from your test kit, then the tank size in US gallons, the temperature and the pH. For a fishless cycle, add the strength printed on your ammonia bottle for a better dose.
- Read the phase, the days estimate, the ammonia toxicity index and any water change suggestion.
Frequently asked questions
- How long does it take to cycle a fish tank?
- The University of Florida’s extension service says a new biofilter needs six to eight weeks to build up enough bacteria. The day counts here, 21 days from Phase 1, 14 from Phase 2 and 7 from Phase 3, are typical durations for each stage, not a forecast for your tank.
- When is it safe to add fish?
- One reading cannot tell you. Phase 4 means ammonia and nitrite read near zero today, which a new tank filled with tap water that contains nitrate can also show. A common hobby check for a fishless cycle is to dose ammonia to about 2 ppm and see both ammonia and nitrite back at zero within 24 hours, then do a large water change to bring nitrate down before stocking.
- Why is ammonia more dangerous at high pH?
- Test kits measure total ammonia, and only the free NH3 part of it is highly toxic; the ionized form, NH4+, is about 100 times less toxic. The free share rises steeply with pH: at 77°F it is 0.57% at pH 7.0 and 5.45% at pH 8.0, so the same 2 ppm reading holds almost ten times as much free ammonia.
- What ammonia and nitrite levels harm fish?
- The University of Florida fact sheet says fish tissue can be damaged whenever free ammonia is above 0.05 mg/L (ppm), and that nitrite is toxic to fish at levels as low as 0.10 mg/L. The fish-in setting here uses a stricter 0.02 ppm of free ammonia as its 100% mark.
- How much ammonia do I add for 2 ppm?
- Enter the percentage on your bottle in Ammonia solution strength. The dose is 2 × litres ÷ (strength × 10) mL, taking the percentage as weight per volume: a 20-gallon tank (75.7 litres) needs 3.0 mL of a 5% solution or 1.5 mL of 10%. Without a strength the card shows a rough 5.0 mL (gallons × 2 ÷ 8). Labels differ on whether the percentage is ammonia or ammonium, so add the dose in steps and test after the water has mixed until the kit reads about 2 ppm.
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