Tank cycling guide

Six weeks of doing almost nothing, which is the hardest part of the hobby. Here is what the numbers should look like while you wait.

Five stages.

Day 0–3

Ammonia added

NH₃ 2–4 ppm

You are feeding bacteria that do not exist yet. Nothing looks like it is happening.

Day 4–12

Nitrite appears

NO₂ rising

The first bacteria colony is working. Ammonia starts to fall.

Day 12–25

Nitrite spike

NO₂ 2–5 ppm

The longest stretch, and where most people give up and add fish.

Day 25–40

Nitrate only

NO₃ 10–20 ppm

Ammonia and nitrite both read zero within 24 hours. That is the finish line.

Day 40+

Stock slowly

Test daily

A third of the final stock, then wait two weeks. Then the next third.

Test every second day and write the numbers down. A cycle you cannot see on paper is a cycle you cannot trust.