How to cycle a new aquarium, day by day
Cycling is not a waiting period you endure. It is a measurable process with two stages, and you can watch both of them happen if you test and log consistently.
A new tank has water, filter media and no bacteria that matter. “Cycling” means growing two colonies: one that turns ammonia into nitrite (Nitrosomonas and relatives), and one that turns nitrite into nitrate (Nitrobacter and relatives). Until both exist, ammonia from fish waste has nowhere to go.
Why fishless instead of “just add hardy fish”
Fish-in cycling works, and it is how most of us started. It also exposes animals to ammonia and nitrite concentrations that damage gills permanently — often without visible symptoms. Fishless cycling replaces the fish with a measured ammonia source, which is faster to control, easier to interpret, and does not require you to sacrifice anything.
The short version
- Fill the tank, dechlorinate, set the heater to the temperature you will run, and start the filter. Nothing else.
- Dose ammonia to 2 ppm (a plain ammonium chloride solution, dosed by the label — not fish food, not a dead shrimp).
- Test ammonia and nitrite every day. Log both. This is the part people skip, and it is the only part that tells you anything.
- When ammonia drops to zero within 24 hours and nitrite is climbing, you are in stage two. Keep dosing ammonia to 1–2 ppm.
- You are cycled when 2 ppm ammonia disappears within 24 hours and nitrite reads zero, two days in a row. Nitrate will be high — that is the point.
- Do a large water change (50–80%) to bring nitrate down, then add fish gradually, not all at once.
What the numbers look like over four to six weeks
| Stage | Ammonia | Nitrite | Nitrate | What it means |
|---|---|---|---|---|
| Day 0–3 | 2 ppm (dosed) | 0 | 0 | Nothing is consuming ammonia yet. |
| Day 4–10 | Falls to 0 in 24h | Rising | Low | Ammonia oxidisers are established. |
| Day 10–25 | 0 after dosing | Spikes, then falls | Rising | Nitrite oxidisers are catching up. |
| Day 25–40 | 0 in 24h | 0 in 24h | 20–80 ppm | Cycled. Nitrate is your water change. |
Three weeks is fast. Six weeks is normal. Eight weeks happens when the tank is cold, the pH is low, or the KH is near zero — the bacteria you need are slower below 24 °C (75 °F) and in soft, acidic water. Logging daily is how you tell “slow” apart from “stalled”.
The three things that stall a cycle
- pH crash. Nitrification consumes alkalinity. If KH is low and pH falls under 6.0, the bacteria stop working. Test KH, not just pH.
- Too much ammonia. Above roughly 4–5 ppm, nitrite oxidation is inhibited and the cycle drags. Dose 2 ppm, not “a capful”.
- Filter off or media replaced. Bacteria live in the media. Rinsing it in tap water or swapping cartridges restarts the clock.
How to log a cycle properly
A cycle is the one time in the hobby when daily numbers matter more than weekly ones. Log ammonia, nitrite, nitrate and temperature every day in the water test log, and note the dose each time you add ammonia. Two weeks of entries will show you the exact day nitrite started falling — which is the day the tank turned the corner, and the day you would otherwise have given up and bought bottled bacteria.
If you keep the log through the first month, you also end up with a baseline: what this tank looks like when it is stable. That baseline is what makes the next problem obvious.
Frequently asked
Can I speed up a cycle?
Do I need to dose ammonia if I use substrate that releases it?
Should I do water changes during the cycle?
Ammonia spike: what to do in the first 24 hours
A step-by-step response to a positive ammonia reading — what to change, what not to do, and the two numbers that decide whether you need to act tonight.
Read →How often should you change aquarium water?
Interval and volume by tank size and stocking — plus the nitrate maths that turns “about 25% weekly” into a schedule you can defend with your own numbers.
Read →9 best aquarium fish for beginners (that stay small)
Small, hardy, forgiving species that work in a 5- to 20-gallon tank — with the parameters each one actually needs and the mistakes each one punishes.
Read →Stop guessing what your tank did last week
Log a test in twenty seconds, get reminded before maintenance is overdue, and keep a history you can actually read back. Free for one tank — no account, no subscription.
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