Aquarium Nitrogen Cycle Calculator
This calculator estimates the daily ammonia (total ammonia nitrogen, TAN) load produced by the fish in your aquarium, based on total fish biomass. The excretion rate used is 0.48 mg TAN per gram of fish per day, a commonly cited mid-range value from aquatic toxicology literature. Enter the total weight of all fish in your tank and the tank volume to see the daily TAN production, the resulting concentration in mg/L, and whether the load is within safe limits for a cycled filter. Results are estimates and depend on feeding rate, temperature, and fish species.
Ammonia load formula
Daily TAN (mg/day) = total fish biomass (g) x 0.48 mg/g/day
Concentration (mg/L/day) = daily TAN / tank volume (L)
The excretion coefficient of 0.48 mg TAN per gram of fish per day is derived from aquatic toxicology studies on freshwater fish held at moderate temperatures and fed a standard diet. The US EPA (1999) Ambient Water Quality Criteria for ammonia describes safe TAN thresholds in relation to pH and temperature.
Nitrogen cycle stages
- Stage 1: Fish excrete ammonia (NH3/NH4+). Ammonia rises (week 1-2).
- Stage 2: Nitrosomonas bacteria colonise and oxidise ammonia to nitrite. Ammonia falls; nitrite rises (week 2-4).
- Stage 3: Nitrospira bacteria colonise and oxidise nitrite to nitrate. Both ammonia and nitrite reach zero (week 4-8).
- Maintenance: Nitrate accumulates and is removed by 20-30% weekly water changes.
- Safe levels: Ammonia 0 mg/L, Nitrite 0 mg/L, Nitrate below 20 mg/L for most species.
Aquarium nitrogen cycle calculator: frequently asked questions
What is the nitrogen cycle in an aquarium?
The nitrogen cycle describes how fish waste (ammonia) is converted by beneficial bacteria. Nitrosomonas bacteria oxidise ammonia (NH3) to nitrite (NO2-), and Nitrospira bacteria oxidise nitrite to nitrate (NO3-). Nitrate is far less toxic and is removed by water changes. A cycled aquarium keeps ammonia and nitrite at or near zero.
How much ammonia does a fish produce?
According to published aquatic toxicology research, fish excrete approximately 0.45-0.50 mg of ammonia-nitrogen (TAN) per gram of body weight per day when fed a standard diet at normal temperatures. This calculator uses 0.48 mg TAN per gram of fish per day as a mid-range estimate.
What is a safe ammonia level in an aquarium?
The US EPA and published aquatic toxicology guidelines indicate that total ammonia nitrogen (TAN) above 0.02 mg/L (as NH3) is harmful to most freshwater fish. In practice, aquarists aim to keep ammonia at 0 mg/L in an established tank. Levels above 0.50 mg/L as TAN can cause acute stress.
How often should I do water changes when cycling?
During the nitrogen cycle (typically 4-8 weeks), partial water changes of 25-30% every 2-3 days help keep ammonia below harmful levels while allowing beneficial bacteria to colonise. The EPA guideline for acute ammonia exposure is 0.02 mg/L of un-ionised NH3.
Why does pH and temperature affect ammonia toxicity?
Ammonia exists in two forms: ionised (NH4+, relatively non-toxic) and un-ionised (NH3, highly toxic). Higher pH and higher temperature shift the equilibrium toward the more toxic NH3 form. A total ammonia reading of 1 mg/L at pH 8 and 25 degrees C is far more dangerous than the same reading at pH 7 and 20 degrees C.
Official sources
- US EPA: Aquatic Life Ambient Water Quality Criteria for Ammonia (Freshwater).
- US EPA (1999): Ambient Aquatic Life Water Quality Criteria for Ammonia (1999).
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.