What really happens in a new tank's first month, and how to cycle safely before adding any livestock.
By Zhao Min · 2026-09-06
Every breath your fish take and every flake they eat ends up as ammonia in the water. Ammonia (NH3) is acutely toxic: at 0.25 ppm it burns gills and skin, at 1 ppm it kills most fish within days. In a river this waste is diluted endlessly; in a glass box, nothing removes it unless the nitrogen cycle does.
The cycle is a relay between two groups of bacteria. First, ammonia-oxidising bacteria (Nitrosomonas and relatives) convert ammonia into nitrite (NO2-). Nitrite is barely less toxic — it binds to haemoglobin and suffocates the fish. Then nitrite-oxidising bacteria (Nitrospira) convert nitrite into nitrate (NO3-), which is safe below roughly 40 ppm and is removed by water changes and consumed by plants.
A mature filter is not a gadget; it is a living colony of these bacteria. Every cycle decision you make — how fast you stock, when you rinse media, whether you replace all your cartridges at once — either feeds or starves that colony.
Week 1 — silent start. Ammonia begins to appear (from a pinch of fish food or a bottled ammonia dose), but no bacteria are established yet. Nothing seems to happen. This is normal; the bacteria are being recruited from the air, the plants, and any seed material you added.
Week 2 — ammonia spike. Ammonia climbs, often past 2 ppm, then starts falling as the first bacterial colony establishes. Nitrite appears at the end of this phase. This is the most dangerous window for any fish in the tank, which is why fishless cycling exists.
Week 3 — nitrite spike. Nitrite rises sharply (a nitrite test kit turns deep purple), sometimes persisting for a week or more, because the second bacterial colony grows more slowly than the first. Keep testing and keep doing water changes to keep both toxins survivable for seed bacteria.
Week 4–6 — the finish line. Nitrite falls to zero and stays there; nitrate climbs steadily. When a 2 ppm ammonia dose converts to nitrate within 24 hours, the cycle is complete. Do a large water change to reset nitrate, and stock slowly.
Fishless cycling grows the bacteria without risking a single fish. Set up the tank fully — filter, heater, substrate, plants, light on a timer — and add an ammonia source: pure household ammonia (no surfactants or fragrances) dosed to 2 ppm, or a pinch of fish food every other day.
Test every two or three days. When ammonia falls below 0.5 ppm, re-dose to 2 ppm; this feeds the colony and speeds establishment. Expect the full run to take three to six weeks depending on temperature (24–28 °C speeds it; below 18 °C it crawls), seeding, and oxygen.
Seeding shortcuts: a cup of filter sludge or a squeezed sponge from an established tank can cut the timeline by a third. Bottled bacterial products help but are inconsistent between brands — treat them as accelerants, not guarantees.
Use a liquid (reagent) test kit, not strips — strips miss the 0.25 ppm readings that matter most during cycling. Always test at the same time of day, and log every reading; the log, not any single number, shows you the trend.
Key ratios tell the story: ammonia falling while nitrite rises means phase 3 is underway; both falling together means the cycle is nearly done; nitrate appearing confirms the full relay is running. If ammonia and nitrite both read zero two days after a dose, you are cycled — verify with one more full-dose test before adding fish.
Three to six weeks at 24–28 °C with a fishless method; longer in cold water, shorter with seed material from an established filter. The real answer is in your test kit: the tank is cycled when a 2 ppm ammonia dose converts fully to nitrate within 24 hours.
You can, but it risks the fish: they will be exposed to ammonia and nitrite spikes. If you must, stock very lightly with hardy fish, test daily, and change 25–50% of the water whenever ammonia or nitrite exceeds 0.25 ppm, dosing a water conditioner that detoxifies ammonia between changes.
Three things: tap water (chlorine and chloramine are disinfectants — rinse media only in old tank water), letting the filter dry out (bacteria die within hours without flow and moisture), and replacing all media at once (you discard most of the colony). Rinse in tank water and replace media in stages, one third at a time.
A mature cycle breaks when something killed the bacteria (medication, tap-water rinse, dried media) or overwhelmed them (overfeeding, a dead fish rotting unseen, a filter stalled overnight). Test, find and remove the cause, do a 50% water change, and dose a detoxifier until the colony recovers over the following days.
No, but they change the picture favourably. Healthy fast-growing plants absorb ammonia directly and can carry a lightly stocked, heavily planted tank through its early weeks — the classic 'silent cycle'. They do not, however, replace the filter colony in a normally stocked tank, and they cannot rescue an overloaded one.
Nitrite (NO2-) is the middle product of the cycle and highly toxic — it prevents the fish's blood from carrying oxygen. Nitrate (NO3-) is the end product and over a hundred times less toxic; it is controlled with weekly water changes and plant uptake. In tests: purple-pink means nitrite (danger), orange-red means nitrate (manage below 40 ppm).