Cycling a New Aquarium
How to establish the beneficial bacteria that make a tank safe for fish — the single most important step before adding any livestock.
Also known as: Nitrogen cycle setup, Fishless cycling
Cycling is the process of growing colonies of beneficial bacteria that convert toxic fish waste into safer compounds. Doing it before adding fish — a fishless cycle — is the most important step in setting up a tank, and skipping it is the most common cause of early fish loss.
What it is
A brand-new tank has no way to process the ammonia that fish waste and uneaten food produce, and ammonia is toxic. Cycling grows the bacteria that handle it, establishing the biological filter that keeps water safe. Until a tank is cycled, adding fish exposes them to ammonia and nitrite poisoning — the classic "new tank syndrome."
What you're growing. Two groups of bacteria colonise the filter and surfaces: one converts ammonia to nitrite, another converts nitrite to nitrate. Ammonia and nitrite are highly toxic; nitrate is far less so and is removed by water changes. A cycle is complete when the filter can take ammonia to zero and nitrite to zero on its own.
Fishless method (recommended). Set up the tank fully — filter running, heater on, water dechlorinated — then add a source of ammonia (bottled ammonia or a pinch of fish food to rot). Test daily. You'll see ammonia rise, then nitrite rise as the first bacteria establish, then both fall to zero as nitrate appears. This typically takes four to eight weeks. Seeding the filter with media or substrate from an established, healthy tank can speed it up considerably.
Do not rely on "fish-in" cycling with live fish unless unavoidable; it is stressful and can be lethal, and requires frequent water changes to keep toxins low. Once ammonia and nitrite both read zero and nitrate is present, do a partial water change and add fish gradually, a few at a time, so the bacteria can keep pace with the rising waste load.
Worked example
A keeper fills a new 75 L tank, dechlorinates, and starts the filter and heater. They dose bottled ammonia to about 2 ppm and test daily. Week 1–2: ammonia climbs, nitrite appears. Week 3–4: ammonia drops to 0, nitrite peaks then falls, nitrate rises. When a fresh dose of ammonia clears to 0 ammonia and 0 nitrite within 24 hours, the tank is cycled. They change 50% of the water and add their first small group of fish.
Related entries
Sources & further reading
- The Nitrogen Cycle in the Aquarium — The Spruce Pets (article)
- Aquarium Nitrogen Cycle — Aqueon (article)