How to cycle a new aquarium

Updated 2026-06-146 min read

Cycling a new aquarium means growing the beneficial bacteria that convert toxic ammonia into safe nitrate before any fish go in. A fishless cycle does this without putting a living animal through the dangerous ammonia and nitrite spikes.

This guide gives you a practical, repeatable fishless cycle: add an ammonia source, test daily, and wait for the tell-tale pattern that says your tank is ready.

Steps

Set up the tank and start the filter
Fill the tank with dechlorinated water, run the filter and heater, and bring the temperature to about 25–27°C. Warmth speeds the bacteria’s growth.
Add an ammonia source
Dose pure household ammonia (no surfactants or perfumes) to reach roughly 2–4 ppm. This feeds the bacteria you are trying to grow. Re-dose to keep ammonia present.
A bottled bacterial starter or a handful of media from an established, healthy tank can cut cycling time significantly.
Test daily and track the numbers
Measure ammonia, nitrite, and nitrate each day. You will watch ammonia rise then fall, nitrite rise then fall, and nitrate steadily climb — the cycle taking hold.
Log water test
pH8.1
Ammonia0.0
Nitrite0.0
Save reading
Wait for ammonia and nitrite to clear
The cycle is nearly done when a fresh dose of ammonia drops to 0, and nitrite also reads 0, within 24 hours. This confirms both bacterial colonies are established.
Do a large water change
Cycling leaves nitrate high. Before adding fish, do a big water change to bring nitrate down to a safe level — under about 20–40 ppm.
Add fish gradually
Stock a small group first and add more over the following weeks. Sudden full stocking can outpace the bacteria you just grew and trigger a mini-spike.