Aquarium Cycling Timeline Generator
Cycling — growing the bacteria that convert toxic ammonia to nitrite and then to relatively harmless nitrate — typically takes 4–8 weeks from a standing start. Enter your start date and method, and this tool lays out the expected milestones with real dates, so you know what your test kit should be showing and when.
Generate your timeline
Timelines are typical ranges at 24–28 °C with pH > 7 and a dechlorinated water source. Cold water, low pH and chloramine slow everything down.
What’s actually happening in the filter
“Cycling” establishes two bacterial guilds on your filter media and surfaces. The first (Nitrosomonas and relatives) oxidises ammonia (NH3/NH4+) — excreted by fish and produced by decaying food — into nitrite (NO2−). The second (Nitrospira and relatives) oxidises nitrite into nitrate (NO3−). Ammonia and nitrite are both toxic to fish at fractions of a ppm; nitrate is only weakly harmful and is removed by water changes and plants. The process is just ecology: each population grows only after its food source appears, which is why nitrite peaks after ammonia falls, and why the second phase usually takes the longest.
Fishless cycling, step by step
- Set up the tank fully — filter running, heater at 24–28 °C, dechlorinated water.
- Dose pure ammonia (ammonium chloride sold for aquariums, or unscented household ammonia with no surfactants) to 2–4 ppm. Fish food works too, but messily and less controllably.
- Test ammonia and nitrite every day or two. Re-dose ammonia back to ~2 ppm whenever it drops below 1.
- When both ammonia and nitrite go from 2 ppm to zero within 24 hours on consecutive days, the tank is cycled.
- Do a large water change to bring nitrate down (often 50–90% after a cycle), match temperature, and add your first fish within a day or two — an unfed cycle starves quickly.
Why we don’t recommend fish-in cycling
Cycling with fish in the tank exposes them to ammonia and nitrite for weeks — both damage gills at low concentrations, and the harm is often invisible until later. It also constrains you: you must hold ammonia below ~0.5 ppm with frequent water changes, which slows the cycle further. If you already have fish in an uncycled tank (it happens), test daily, change water whenever ammonia or nitrite exceeds ~0.25–0.5 ppm, add a bottled nitrifier product, and feed lightly. For fish showing ongoing distress, seek advice from an experienced aquarist or aquatic veterinarian — this page is general husbandry information, not veterinary guidance.
Frequently asked questions
Do bacteria-in-a-bottle products actually work?
The reputable ones genuinely shorten cycles — often to 1–3 weeks — though results vary with product freshness and storage. None of them make a tank instantly safe by themselves: dose, then verify with your own ammonia/nitrite tests before trusting fish to it.
My cycle seems stalled — nitrite has been maxed out for two weeks. Is it broken?
Usually not. Off-the-chart nitrite is normal in phase 3 and the test can’t distinguish 5 ppm from 15. Check pH hasn’t crashed below ~6.5 (nitrification stalls in acid water — a partial water change fixes it) and confirm you aren’t overdosing ammonia above ~4 ppm. Then wait; the nitrite-eaters are just slower.
Can I speed up cycling?
The big levers: seed with media or substrate from an established healthy tank (the single most effective trick — it can cut the timeline in half or more), keep water at 26–28 °C, keep pH above 7, and ensure good oxygenation. Nothing reputable gets a bare tank done in a day.
Does a planted tank need cycling?
Heavily planted tanks partly self-cycle: plants consume ammonia directly, and aquasoils both leach and adsorb it. Many planted-tank keepers stock lightly after 2–4 weeks of plant growth without a formal cycle — the “silent cycle”. It works with genuinely heavy planting; with three small plants it does not.