How to Cycle a New Aquarium Before Adding Fish
Cycling builds the bacteria that keep fish safe. Follow this simple walkthrough before you buy your first fish.
Before fish move in, a new tank needs a colony of helpful bacteria. This process is called cycling, and skipping it is one of the most common causes of early fish loss.
What cycling means
Fish waste produces ammonia, which is toxic. Beneficial bacteria convert ammonia into nitrite, which is also toxic, and then into nitrate, which is far less harmful and removed with water changes.
Set up the tank
- Install the filter, heater and substrate, and fill with water treated with a dechlorinator.
- Run the filter continuously so bacteria can establish.
- Buy a liquid test kit that measures ammonia, nitrite and nitrate.
Cycle without fish
Add a source of ammonia according to the product directions, then test every few days. Ammonia will rise and then fall as nitrite appears. Later, nitrite falls and nitrate rises.
When the tank is ready
The tank is cycled when it processes ammonia and nitrite down to zero within a day while nitrate is present. This often takes several weeks. Do a partial water change to reduce nitrate, then add fish gradually, a few at a time, and keep testing.
This article is general pet care information and not a substitute for advice from a licensed veterinarian.
