The Nitrogen Cycle
Before a single fish goes in, a tank needs its own invisible ecosystem of bacteria. This is what "cycling" actually means, why it takes time, and how to tell it is done.
Key facts
- Ammonia and nitrite should both read 0 before fish go in — close to universal agreement across sources.
- A full unseeded cycle commonly takes 2-6 weeks; seeding with mature filter media from an established tank can shorten this to roughly 1-2 weeks.
- There is no single "safe" nitrate number — it depends on what you keep; general community-tank guidance often clusters around 20-40 ppm, and sources disagree on the exact ceiling.
The chain: ammonia to nitrite to nitrate
Fish waste and uneaten food break down into ammonia, which is toxic to fish. One group of bacteria oxidizes ammonia into nitrite, which is also toxic. A second group of bacteria oxidizes nitrite into nitrate, a far less dangerous compound removed by water changes and by plants. These two bacterial populations, colonizing filter media and other surfaces in the tank, are what a "cycle" actually establishes.
How long it takes
An unseeded fishless cycle, dosing pure ammonia, commonly takes 2 to 6 weeks: the cycle is considered complete once a 24-hour dose of ammonia is fully converted to nitrate, with ammonia and nitrite holding at zero. Nitrite-consuming bacteria typically establish more slowly than ammonia-consuming bacteria, so the high-nitrite phase often outlasts the ammonia phase.
Seeding with mature filter media or substrate from an already-cycled tank can shorten this to roughly 1-2 weeks. Live plants also take up some ammonia and nitrate directly, though they do not replace the bacterial colony.
What "safe" numbers actually look like
Ammonia at 0 ppm and nitrite at 0 ppm are targets essentially every source agrees on. Nitrate is different: there is no single agreed ceiling, because it depends on what you keep. General community-tank guidance often clusters around 20-40 ppm; sensitive or wild-caught species (discus, for example) are often kept well below that, while some hardy livebearers and cichlids tolerate the higher end. Several sources flag nitrate above roughly 40 ppm as unsafe.
Common beginner mistakes
- Adding too many fish at once before the bacterial colony is established.
- Rinsing filter media under tap water — chlorine kills the bacteria; rinse it in removed tank water instead.
- Cleaning the filter or decor too thoroughly, stripping the biofilm the bacteria actually live on.