The Nitrogen Cycle
What's actually happening inside your tank during those weeks before your axolotl moves in — and why you can't rush it.
Three compounds, two colonies of bacteria
Everything about cycling comes down to this chain. Get familiar with it and every test result starts to make sense.
Nitrosomonas
Nitrospira
Where do the bacteria actually live?
Not floating free in the water — almost all of them are colonising surfaces, and your filter media is the biggest surface in the tank.
Both types of bacteria are slow to reproduce — under good conditions they roughly double their numbers every 12–24 hours, not minutes like most bacteria you'd think of. You're starting from a near-zero population and growing it from scratch.
That's why fishless cycling exists in the first place. Dosing ammonia manually — with nothing living in the tank yet — lets you feed that growth safely, at a controlled level, for as long as it takes. Nobody's gills are exposed to the toxic middle stages while the colony builds.
Temperature, pH and a stable source of carbonate hardness (KH) all affect how fast the bacteria can work. 26–29°C is the sweet spot for nitrifying bacteria — keep the tank there for the whole cycling process and you'll get through it as fast as it's possible to go. Drop toward axolotl temperature too early and everything slows right down, sometimes doubling the wait.
That's the whole strategy in one line: run it warm to cycle fast, then bring the temperature down gradually — no more than 1–2°C a day — once the bacteria are established, before your axolotl goes anywhere near it.
Bicarbonate of soda is the cheap, easy fix for KH while you're cycling — nothing's living in the tank yet to be fussy about it. But it only lifts KH, not GH. If your tap water is naturally soft on both fronts, bicarbonate alone isn't a long-term plan once your axolotl's in — you'll want a proper remineralising buffer that brings KH and GH up together.
The three stages you'll actually see
Stage one
Ammonia goes in, and for a while, nothing happens — the first colony is establishing. Once it's grown enough, ammonia starts clearing within a day of dosing.
Stage two
Nitrite appears and climbs, often off the top of the test card. This looks alarming — it isn't. It just means the first colony is outpacing the second. Small top-up doses only from here.
Stage three
Both colonies are established. Confirm it with three days running: dose 4 ppm, and both ammonia and nitrite need to hit zero within 24 hours, every time.
Ready to see it in action?
Play with a simulated tank, track your real readings day to day, or run a quick one-off check — all right here.
Play with a cycle
A model tank, sped up. Watch what your filter would actually be doing — no waiting weeks for real.
Track your real cycle
Log your actual test kit readings as you go. This tells you exactly what to dose next, and holds you to the three-day confirm test properly.