A newly filled aquarium contains no beneficial bacteria. Add fish before those bacteria are established and ammonia from fish waste will accumulate to toxic levels within days. The nitrogen cycle — the biological process that converts ammonia to less harmful compounds — must complete before the tank is safe for livestock. This typically takes three to six weeks using the fishless method described here.
What the nitrogen cycle actually does
Fish excrete ammonia (NH₃) directly through their gills and via decomposing waste. In an uncycled tank, ammonia accumulates. Two groups of bacteria deal with it sequentially. Nitrosomonas species oxidise ammonia into nitrite (NO₂⁻). A second group — primarily Nitrospira — then converts nitrite into nitrate (NO₃⁻). Nitrate is far less toxic and is removed through regular water changes.
Both groups colonise the surfaces inside your filter media, on substrate grains, and on tank décor. The filter contains the highest concentration because it receives the most oxygen-rich water flow. This is why rinsing filter media in tap water — which contains chlorine — destroys the cycle and causes ammonia spikes even in established tanks.
Nitrosomonas and Nitrospira are not free-floating in the water column. They attach to surfaces. Filter media surface area is the limiting factor for how much bacterial capacity a tank can support.
Equipment needed before you start
- Liquid test kit (API Freshwater Master Test Kit or equivalent) — strips are not accurate enough for cycling
- Pure ammonia — hardware-store ammonia with no surfactants, dyes, or perfumes; shake the bottle: foam that disappears within seconds means no surfactants
- Dechlorinator — treat tap water before each top-up; chlorine and chloramine both kill cycling bacteria
- Running filter and heater — temperature between 26–28°C accelerates bacterial growth by 20–30% compared to 20°C
- A log or spreadsheet — recording daily test results is the only reliable way to track progress
Step-by-step fishless cycling
Day 1 — Fill and dechlorinate
Fill the tank with tap water. Add dechlorinator at the dose specified for your tap water volume — not the tank volume, since a 100-litre tank typically holds 80–85 litres when décor and substrate are included. Run the filter. Set the heater to 27°C. Allow 24 hours for the water to reach temperature before adding ammonia.
Day 2 — Dose ammonia to 2–4 ppm
Add pure ammonia until your test reads 2–4 mg/L (ppm). For most pure ammonia solutions at 10% concentration, this is approximately 1 ml per 40 litres. Test immediately after dosing and adjust. Record the reading. Do not exceed 4 ppm — higher concentrations slow bacterial colonisation rather than accelerating it.
Days 3–14 — Maintain ammonia, watch for nitrite
Test ammonia daily. When it drops below 1 ppm, redose to 2 ppm. The drop confirms Nitrosomonas are active. Within the first two weeks you should also see nitrite appearing on tests. Early nitrite readings of 0.25–1 ppm are a positive sign — it means ammonia is being processed.
Days 14–28 — Nitrite peak and nitrate emergence
Nitrite will climb, sometimes exceeding the top of the test scale (5 ppm). This is normal. Continue dosing ammonia to maintain 1–2 ppm. As Nitrospira establish, nitrate will begin rising on tests. Nitrite will start dropping. This phase commonly lasts one to two weeks.
Days 28–42 — Final verification
The cycle is complete when all three of the following are true simultaneously:
- Ammonia reads 0 ppm within 24 hours of a 2 ppm dose
- Nitrite reads 0 ppm within 24 hours of the same dose
- Nitrate is present and rising between water changes
Perform a 50% water change to reduce nitrate below 20 ppm, then add fish.
Reading test results: reference ranges
| Parameter | During cycling | Cycle complete (target) |
|---|---|---|
| Ammonia (NH₃/NH₄⁺) | 0.25–4 ppm (variable) | 0 ppm within 24 h of 2 ppm dose |
| Nitrite (NO₂⁻) | 0–5+ ppm | 0 ppm within 24 h |
| Nitrate (NO₃⁻) | Rising from 0 | Present; below 20 ppm after water change |
| pH | Monitor; keep above 7.0 | Stable; 6.8–7.8 suits most species |
pH below 7.0 significantly slows bacterial activity. If your tap water is acidic, add a small amount of crushed coral or a pH buffer to maintain cycling conditions above neutral.
Common mistakes that stall the cycle
- Rinsing filter media under tap water — always use tank water for any filter maintenance during and after cycling
- Over-dosing ammonia — concentrations above 8 ppm inhibit bacterial growth; if you overshoot, do a water change to dilute
- Skipping dechlorinator on top-ups — even a single top-up with untreated tap water can set the cycle back by several days
- Low water temperature — below 20°C bacterial reproduction slows noticeably; below 15°C the cycle may stall entirely
- Using antibiotics or ich medication before cycling completes — many medications kill the cycling bacteria along with pathogens
Accelerating with bacterial seeding
Established filter media from a healthy tank transfers live bacteria directly. A handful of used filter sponge or a few weeks' worth of used ceramic bio-rings placed in your filter can reduce cycling time from six weeks to as little as ten days. The donor tank must have zero nitrite readings before media is transferred, and the seeding must happen within the hour — bacteria begin dying without oxygenated water flow.
Bottled bacterial supplements (Seachem Stability, Tetra SafeStart Plus) are less reliable than seeding with live media, but they do modestly accelerate the early stages when used correctly. Follow manufacturer dosing and combine them with normal ammonia maintenance.
Further reading
For detailed biochemistry of the nitrification process, the ScienceDirect topic page on nitrification provides peer-reviewed background. The FishBase database lists water parameter requirements for each species and is useful for verifying compatibility before purchasing.